Astronomy basics

Astronomy basicsLight, color, andthe electromagnetic spectrumSee chart below

1. What is the electromagnetic spectrum?

The electromagnetic spectrum consists of all the different wavelengths of electromagnetic radiation, including light, radio waves, and X-rays (see chart at the bottom of this page). It is a continuum of wavelengths from zero to infinity. We name regions of the spectrum rather arbitrarily, but the names give us a general sense of the energy; for example, ultraviolet light has shorter wavelengths than radio light. The only region in the entire electromagnetic spectrum that our eyes are sensitive to is the visible region.

Gamma rays have the shortest wavelengths, of less than 0.01 nanometers (about the size of an atomic nucleus). This is the highest frequency and most energetic region of the electromagnetic spectrum. Gamma rays can result from nuclear reactions taking place in objects such as pulsars, quasars, and black holes.

X-rays range in wavelength from 0.01 to 10 nanometers (about the size of an atom). They are generated, for example, by super-heated gas from exploding stars and quasars, where temperatures are near a million to ten million degrees.

Ultraviolet radiation has wavelengths of 10 to 310 nanometers (about the size of a virus). Young, hot stars produce a lot of ultraviolet light and bathe interstellar space with this energetic light.

Visible light covers the range of wavelengths from 400 to 700 nanometers (from the size of a molecule to a protozoan). The Sun emits most of its radiation in the visible range, which our eyes perceive as the colors of the rainbow. Our eyes are sensitive only to this small portion of the electromagnetic spectrum.

Infrared wavelengths span from 710 nanometers to 1 millimeter (from the width of a pinpoint to the size of small plant seeds). At a temperature of 37 degrees C, our bodies radiate with a peak intensity near 900 nanometers.

Radio waves are longer than 1 millimeter. Since these are the longest waves, they have the lowest energy and are associated with the lowest temperatures. Radio wavelengths are found everywhere: in the background radiation of the universe, in interstellar clouds, and in the cool remnants of supernova explosions, to name a few. Radio stations use radio wavelengths of electromagnetic radiation to send signals that our radios then translate into sound. These wavelengths are typically a few feet long in the FM band and up to 300 yards or more in the AM band. Radio stations transmit electromagnetic radiation, not sound. The radio station encodes a pattern on the electromagnetic radiation it transmits, and then our radios receive the electromagnetic radiation, decode the pattern and translate the pattern into sound.

New instrumentation and computer techniques of the late 20th century allow scientists to measure the universe in many regions of the electromagnetic spectrum. We build devices that are sensitive to the light that our eyes cannot see. Then, so that we can "see" these regions of the electromagnetic spectrum, computer image-processing techniques assign arbitrary color values to the light.

2. What is a light wave?

Light is a disturbance of electric and magnetic fields that travels in the form of a wave. Imagine throwing a pebble into a still pond and watching the circular ripples moving outward. Like those ripples, each light wave has a series of high points known as crests, where the electric field is highest, and a series of low points known as troughs, where the electric field is lowest. The wavelength is the distance between two wave crests, which is the same as the distance between two troughs. The number of waves that pass through a given point in one second is called the frequency, measured in units of cycles per second called Hertz. The speed of the wave therefore equals the frequency times the wavelength.

3. What is the relationship between frequency and wavelength?

Wavelength and frequency of light are closely related. The higher the frequency, the shorter the wavelength. Because all light waves move through a vacuum at the same speed, the number of wave crests passing by a given point in one second depends on the wavelength. That number, also known as the frequency, will be larger for a short-wavelength wave than for a long-wavelength wave. The equation that relates wavelength and frequency is:

For electromagnetic radiation, the speed is equal to the speed of light, c, and the equation becomes:

4. What is the relationship between wavelength, frequency, and energy?

The energy of a wave is directly proportional to its frequency, but inversely proportional to its wavelength. In other words, the greater the energy, the larger the frequency and the shorter (smaller) the wavelength. Given the relationship between wavelength and frequency described above, it follows that short wavelengths are more energetic than long wavelengths.

Electromagnetic spectrum

5. How are wavelength and temperature related?

All objects emit electromagnetic radiation, and the amount of radiation emitted at each wavelength determines the temperature of the object. Hot objects emit more of their light at short wavelengths, and cold objects emit more of their light at long wavelengths. The radiation temperature of an object is related to the wavelength at which the object gives out the most light. We refer to the amount of light emitted at a particular wavelength as the intensity.

When you plot the intensity of light from an object at each wavelength, you trace out a smooth curve called a blackbody curve. For any temperature, the blackbody curve shows how much energy (intensity) is radiated at each wavelength. The wavelength where the intensity peaks determines the color of that the object. The intensity peak will be at shorter (bluer) wavelengths for hotter objects, and at longer (redder) wavelengths for cooler objects. Therefore, you can tell the temperature of a star or galaxy by its color because color is closely related to the wavelength at which its light intensity peaks.

Blackbody curves, for objects of all temperatures, have a similar shape, as shown in the graph below. However, the peak of the curve for a hotter object will be larger (more intense) than will the peak of the curve for a cooler object. For example, the intensity difference between the peak of the curve for an object at 30,000 K and the peak of the curve for an object at 300 K (body temperature) is a factor of 10 billion. This means that a star at 30,000 K puts out more energy by a factor of 10 billion than does a human at body temperature.

Because of the large intensity difference, it would be difficult to show both of these curves on the graph above without using logarithms. To plot blackbody curves with large intensity differences on the Heating Up page of Amazing Space's "Star Light, Star Bright," the scale of the intensity axis adjusts itself for each temperature change.

6. How are temperature and color related?

The amount of light produced by an object at each wavelength depends on the temperature of the object producing the light. Stars hotter than the Sun (over 6000 degrees C) put out most of their light in the blue and ultraviolet regions of the spectrum. Stars cooler than the Sun (below 5000 degrees C) put out most of their light in the red and infrared regions of the spectrum. Solid objects heated to 1000 degrees C appear red but are putting out far more (invisible) infrared light than red light.

7. What information can light reveal about the stars?

Electromagnetic radiation, or light, is a form of energy. Visible light is a narrow range of wavelengths of the electromagnetic spectrum. By measuring the wavelength or frequency of light coming from objects in the universe, we can learn something about their nature. Since we are not able to travel to a star or take samples from a galaxy, we must depend on electromagnetic radiation to carry information to us from distant objects in space.

The human eye is sensitive to a very small range of wavelengths called visible light. However, most objects in the universe radiate at wavelengths that our eyes cannot see. Astronomers use telescopes with detection devices that are sensitive to wavelengths other than visible light. This allows them to study objects that emit this radiation, which would otherwise be invisible to us. Computer techniques then code the light into arbitrary colors that we can see.

The Hubble Space Telescope is able to measure wavelengths from about 0.1150 to 2 micrometers, a range that covers more than just visible light. These measurements of electromagnetic radiation enable astronomers to determine certain physical characteristics of objects, such as their temperature, composition, and velocity.

8. Why does Hubble need to take images using non-visible light?

The human eye is sensitive to a very small range of wavelengths called visible light. However, many celestial objects in the universe radiate at wavelengths that our eyes cannot see and each type of radiation provides clues as to the nature of the object in question. Astronomers study celestial objects with detection devices that are sensitive to wavelengths other than visible light and then use computer techniques that code the light into colors that we can see.

Able to measure wavelengths from about 115 nanometers to 2500 nanometers, the Hubble Space Telescope looks at the energy that is not only visible, but also infrared and ultraviolet. These measurements better enable astronomers to determine physical characteristics of objects, such as their temperature, composition, and velocity.

9. Why do scientists need filters, and how do they work?

Different wavelengths of light provide scientists with different information about the objects they are studying. For instance, infrared light can reveal details about objects shrouded in dust. Infrared light emitted by an object will pass through dust unlike visible light, which is scattered. In contrast, ultraviolet light can reveal details about the stellar wind around stars. (When talking about our sun, this is called the solar wind.) Astronomers have ways of breaking light into a spectrum, which reveals a lot of information (including properties of the source of the light, the material through which the light passes, or the material off of which the light reflects).

However, sometimes scientists want to capture specific ranges of wavelengths of light, so they use a filter. A filter will allow only light within a small range of wavelengths to pass through. When the Hubble Space Telescope takes an image using a filter, that image shows only the varying intensity of light in that small range. In making color pictures, scientists usually use a red filter, a green filter, and a blue filter (the red filter allows light only in the red range to enter, etc.). By combining these images scientists can create full-color pictures.

10. What are the special electronic detectors that the Hubble Space Telescope uses to record light?

Hubble uses professional-grade versions of the same detectors found in a digital camera. Each optical instrument on the telescope has a set of charge-coupled devices (CCDs) composed of a grid of pixels (picture elements) that measure the intensity of light that strikes them. For example, one instrument (the Wide Field and Planetary Camera 2) has four CCDs, each of which contains 640,000 pixels. Each pixel turns the light intensity it measures into a number.

These numbers are systematically downloaded to the Space Telescope Science Institute where they are translated into black-and-white images. Using computers, two or more of these images can be colorized and combined to produce a color image. CCDs are used only for optical light. Ultraviolet and infrared light have different detectors, which are not called CCDs but operate on similar principles.

11. Why choose red, green, and blue as the assigned colors for images?

The colors red, green, and blue are chosen because they are the primary colors of light. By combining these colors of light, white light is produced. Combinations of two of these colors produce other familiar colors: blue + green = cyan, red + blue = magenta, and red + green = yellow.

12. How are the colors assigned to the black and white images?

If red, green, and blue filters have been used, the red filter is assigned red light, the green filter is assigned green light, and the blue filter is assigned blue light. This changes the black-and-white scale into tones of red, green, and blue, respectively. Using computers, these images can be combined into one image, which represents (as close as possible) the true colors of the object being imaged. In general, when other filters are used, blue is assigned to the shortest wavelength light while red is assigned to the longest wavelength, with green being the wavelength in the middle.

13. What objects does the Hubble Space Telescope observe and why?

The Hubble Space Telescope takes observations of almost every part of the sky, as long as it isn't toward the Sun. In our own solar system, the telescope can take incredibly detailed pictures of the outer planets and their moons. A crescent of Venus has been taken with the Hubble Space Telescope and the moon has been shot once, but Mercury is too close to the Sun to be imaged.

Farther out, Hubble takes images of stars and nebulae in our own galaxy as well as in galaxies in the larger universe. The majority of Hubble's time is used for scientific research, taking many observations of specific objects that the scientists are studying. However, a very small portion of Hubble's time is dedicated to taking beautiful pictures for public enjoyment.

GalaxiesSee image below

1. What is a galaxy?

A galaxy is an enormous collection of a few million to trillions of stars, gas, and dust held together by gravity. Galaxies can be several thousand to hundreds of thousands of light-years across.

2. What is the name of our galaxy?

The name of our galaxy is the Milky Way. All of the stars that you see at night and our Sun belong to the Milky Way. When you go outside in the country on a dark night and look up, you will see a milky, misty-looking band stretching across the sky. When you look at this band, you are looking into the densest parts of the Milky Way: the disk and the bulge.

3. Where is Earth in the Milky Way galaxy?

Our solar system is in a spiral arm called the Orion Arm, and is about two-thirds of the way from the center of our galaxy to the edge of the starlight. Earth is the third planet from the Sun in our solar system of nine planets.

4. What is the closest galaxy like our own, and how far away is it?

The closest spiral galaxy is Andromeda, a galaxy much like our own Milky Way. It is 2.2 million light-years away from us. Andromeda is approaching our galaxy at a rate of 670,000 miles per hour. Five billion years from now it may even collide with our Milky Way galaxy.

5. Why do we study galaxies?

By studying other galaxies, astronomers learn more about the Milky Way, the galaxy that contains our solar system. Answers to such questions as "Do all galaxies have the same shape?," "Are all galaxies the same size?," "Do they all have the same number of stars?," and "How and when did galaxies form?" help astronomers learn about the history of the universe. Galaxies are visible to vast distances, and trace the structure of the visible universe with their collections of billions of stars, gas, and dust.

6. What are the parts of a galaxy?

A galaxy contains stars, gas, and dust. In a spiral galaxy like the Milky Way the stars, gas, and dust are organized into a bulge, a disk containing spiral arms, and a halo. Elliptical galaxies have a bulge-shape and a halo, but do not have a disk.

Bulge A round structure made primarily of old stars, gas, and dust. The bulge of the Milky Way is roughly 10,000 light-years across. The outer parts of the bulge are difficult to distinguish from the halo.

Disk A flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The disk of the Milky Way is 100,000 light-years across and 2000 light-years thick. It contains mostly young stars, gas, and dust, which are concentrated in spiral arms. Some old stars are also present.

Spiral arms Curved extensions beginning at the bulge of a spiral galaxy, giving it a "pinwheel" appearance. Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral arms are found only in spiral galaxies.

Halo The halo primarily contains individual old stars and clusters of old stars (globular clusters). It may be over 130,000 light-years across. The halo also contains dark matter, which is material that we cannot see but whose gravitational force can be measured.

Stars, gas, and dust Stars come in a variety of types. Blue stars, which are very hot, tend to have shorter lifetimes than red stars, which are cooler. Regions of galaxies where stars are currently forming are therefore bluer than regions where there has been no recent star formation. Spiral galaxies seem to have a lot of gas and dust, while elliptical galaxies have very little gas or dust.

7. How are galaxies classified? What do they look like?

Edwin Hubble classified galaxies into four major types: spiral, barred spiral, elliptical, and irregular (see also Question 8 and Question 10). Most galaxies are spirals, barred spirals, or ellipticals.

A spiral galaxy consists of a flattened disk containing spiral (pinwheel-shaped) arms, a bulge at its center, and a halo. Spiral galaxies have a variety of shapes, and they are classified according to the size of the bulge and the tightness and appearance of the arms. The spiral arms, which wrap around the bulge, contain many young blue stars and lots of gas and dust. Stars in the bulge tend to be older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy. These galaxies rotate somewhat like a hurricane or a whirlpool. (See a side view of a spiral galaxy, below.)A barred spiral galaxy is a spiral that has a bar-shaped collection of stars running across its center.

An elliptical galaxy does not have a disk or arms; rather, it is characterized by a smooth, ball-shaped appearance. Ellipticals contain old stars and possess little gas or dust. They are classified by the shape of the ball, which can range from round to oval (baseball-shaped to football-shaped). The smallest elliptical galaxies (called dwarf ellipticals) are probably the most common type of galaxy in the nearby universe. In contrast to spirals, the stars in ellipticals do not revolve around the center in an organized way. The stars move on randomly-oriented orbits within the galaxy like a swarm of bees.

An irregular galaxy is neither spiral nor elliptical. Irregular galaxies tend to be smaller objects without definite shape, and they typically have very hot newer stars mixed in with lots of gas and dust. These galaxies often have active regions of star formation. Sometimes their irregular shape is the result of interactions or collisions between galaxies. Observations such as the Hubble Deep Fields show that irregular galaxies were more common in the distant (early) universe.

The Warped Galaxy

8. How are galaxies classified today?

Hubble's "Tuning Fork" Diagram

See reference: http://www.smv.org/hastings/tunfork.htmToday we classify galaxies mainly into two major groups following Hubble's examples, as shown above. Elliptical galaxies range from round shapes (E0) to oval shapes (E7).

Spiral galaxies have a pinwheel shape and are classified according to their bulge, as well as how tightly their arms are wrapped around the bulge. They range from Sa, which has a large bulge and tight, smooth arms, to Sc, which has a small bulge and loose, lumpy arms.

Barred spiral galaxies, classified as SB, are pinwheel-shaped and have a distinct "bar" of stars, dust, and gas across their bulge. They range from an SBa, which has a bar across its large bulge and tight, smooth arms, to an SBc, which has a bar across its small bulge and loose, lumpy arms.

Irregular galaxies have no definite shape but still contain new stars, gas, and dust. The chart below summarizes the properties of the main classes of galaxies.

Properties of the main classes of galaxies

Spiral/Barred Spiral(S/SB)

Elliptical(E)

Irregular(Irr)

Shape and structural properties

Have disks of stars, gas, and dust containing spiral arms that attach to a central bulge. Sa and SBa have the largest bulges. SB galaxies have central bars.

Have neither disk nor arms. Stars are distributed evenly from near circular to oval (football).

Have no definite structure.

Stellar content

Have both young and old stars. Halos consist of old stars only.

Contain mostly old stars.

Contain both young and old stars.

Gas and dust

Disks contain gas and dust. Halos contain little gas or dust.

Have little or no gas or dust.

Have lots of gas and dust.

Star formation

Stars form largely in spiral arms.

Have little or no star formation.

Have lots of star formation.

Stellar motion

Gas and stars rotate around the center of the galaxy.

Stars move on randomly oriented orbits like a swarm of bees.

Stars and gas have irregular orbits.

9. Who is Edwin P. Hubble and what has he to do with galaxies?

Edwin P. Hubble revolutionized cosmology by proving that galaxies are indeed "island universes" beyond our Milky Way galaxy. His greatest discovery was in 1929, when he identified the relationship between a galaxy's distance and the speed with which it is moving. The farther a galaxy is from Earth, the faster it is moving away from us. This is known as Hubble's Law. He also constructed a method of classifying the different shapes of galaxies with the Hubble Tuning Fork (see question 8).

Edwin Powell Hubble was born in Kentucky, where he grew up observing the habits of birds and animals. In 1910 he received his undergraduate degree from the University of Chicago and studied law under a Rhodes Scholarship at Oxford University. Later he changed his mind and completed a Ph.D. in astronomy at Chicago's Yerkes Observatory in 1917. He had several other interests, and for a while he thought of becoming a professional boxer. He also enjoyed basketball, and even answered a call in World War I to serve in the infantry.

Hubble once said that he "chucked the law for astronomy," knowing that even if he was second-rate or third-rate, it was astronomy that mattered.

10. Galaxy names are identified by a group of letters and numbers. What do they stand for?

Scientists classify galaxies in different catalogs. The most common catalog is NGC, which stands for New General Catalog. Other catalogs include M (Messier), ESO (European Southern Observatory), IR (Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog). Sometimes a galaxy appears in more than one catalog and can have more than one name.

The numbers following the letters, such as Mrk917 (Sc) or NGC1433 (SB), indicate the galaxy's entry in the catalog and are often related to the galaxy's relative position in the sky.

11. What are colliding galaxies?

When two or more galaxies are close enough to each other, gravitational forces will pull the galaxies toward each other. This gravitational attraction increases as the galaxies travel toward each other. The galaxies may pass by each other or collide. Two galaxies that are interacting or colliding may be referred to as a pair, or one galaxy may be referred to as a companion of the other.

The Hubble images below show how different colliding galaxies can look. The appearance of an interacting system of galaxies depends on many factors, including the stage of the interaction, the number of galaxies involved in the interaction, their masses and types, how close they are, and how they approach each other.

Examples of Colliding Galaxies

The Antennae

This is a close-up view of two galaxies, probably originally spirals, in the process of merging. The collision has triggered the birth of over 1000 clusters of young stars.

The Cartwheel

A smaller galaxy (not visible here) has passed through the Cartwheel. A "ripple" of star formation (the blue ring) resulted from the collision.

The Polar Ring Galaxy

This unusual structure may be the result of a collision between a large and a small galaxy. Young stars have formed in the ring of gas and dust stripped away from the smaller galaxy. This ring orbits the remains of the larger galaxy.

Stephan's Quintet

Gravitational forces between galaxies in this group have altered some of the galaxies' shapes. The area is littered with stars and gas ripped from the galaxies, as well as new clusters of stars that formed as a result of the interactions.

The Antennae galaxies (upper left) are an example of two spirals that are in the process of colliding. We will not see the end result during our lifetimes because this process takes hundreds of millions of years. Sometimes smaller galaxies plunge into larger galaxies. This type of collision produces a ripple effect, like a rock thrown into a pond. The Cartwheel galaxy (upper right) is an example of this type of collision. The outer ring of blue stars in this galaxy indicates a ripple of star formation resulting from the collision.

Our Milky Way and Andromeda are two spiral galaxies that may eventually collide (about 5 billion years in the future).

12. What happens when galaxies collide?

When galaxies collide, they experience a gravitational pull toward each other. This gravitational pull distorts the shapes of the galaxies, and can pull material from one galaxy to the other. In many cases, the pull of gravity may result in the galaxies merging. The individual stars within the galaxies do not collide since they are so far apart, relative to their size.

Clouds of gas within the galaxies do collide, however. As a result, large amounts of gas become concentrated in one or more areas of the system. As the collision compresses the gas, the gas becomes dense. The clouds of gas collapse under gravitational forces and form large numbers of new stars. This rapid, short-lived episode of star formation activity is referred to as a starburst. Intense starbursts can use up nearly all of the available gas.

13. Do all interacting galaxies merge?

Interacting galaxies do not always merge together to form a single object. Scientists think that some galaxies (such as M51 and its companion, NGC 5195) will simply pass by each other without merging. Galaxies can also pass through each other without merging. The Cartwheel galaxy is believed to be the aftermath of such a pass-through. Some galaxies do merge. The Antennae are thought to represent a merger between two gas-rich spiral galaxies. When the merger is complete, the Antennae may end up looking like a single elliptical galaxy.

14. Why do we study distant galaxies, if they are faint and hard to observe?

When we study astronomical objects, we are actually looking back in time. Light from the Sun takes eight minutes to reach Earth. The light we see today from the next nearest star was emitted about four years ago. Light from the nearest galaxy like our own, Andromeda, takes over 2 million years to reach us. That is, we see Andromeda as it appeared more than 2 million years ago. Observations of distant galaxies show us what the universe looked like at an earlier time in the history of the universe. By studying the properties of galaxies at different epochs, we can map the evolution of the universe.

15. When scientists study these distant galaxies, what do they look at?

They observe many properties of each galaxy, including size, shape, brightness, color, amount of star formation, and distance from Earth. This information helps astronomers to determine how these structures may have formed and evolved.

16. What is a "deep" field?

In astronomical terms, a deep field is a long-exposure observation taken to view very faint objects. Light from these objects is collected over a large period of time, so the detectors have a chance to gather as much light as possible. Objects can be very far away and appear faint to us due to the vast distances over which the light must travel; and/or objects can lie close to us and be faint because they don't give off much light. So "deep" doesn't necessarily mean far. However, in the case of the Hubble Deep Fields (HDFs) and the Hubble Ultra Deep Field (HUDF), deep does mean far away since the images were taken in areas that we know have few nearby stars.

17. What are the Hubble Deep Fields?

The Hubble Deep Field project was inspired by some of the first deep images to return from the Wide Field and Planetary Camera 2 (WFPC2), after it was installed during the 1993 Hubble Space Telescope servicing mission. These images showed that the early universe contained galaxies in a bewildering variety of shapes and sizes. Some had the familiar elliptical and spiral shapes seen among galaxies today, but there were many peculiar shapes as well. Such images of the early universe are likely to be one of the enduring legacies of the Hubble Space Telescope. Few astronomers had expected to see this activity presented in such amazing detail.

Impressed by the results of earlier observations such as the Hubble Medium Deep Survey, astronomers at the Space Telescope Science Institute (STScI), and STScI's Director, Robert Williams, realized that they could provide a service to the entire astronomical community by taking the deepest optical picture of the universe. The research was done by aiming Hubble at a single piece of the northern sky for 10 days (150 consecutive orbits) in December, 1995. Images from the Hubble Deep Field project were made available to the astronomers around the world shortly after completion of the observation.

A few thousand previously unseen galaxies are visible in the original "deepest ever" view of the universe, called the Hubble Deep Field (later named the HDF-North). In addition to the classical spiral and elliptical forms, the variety of other galaxy shapes and colors seen in the image are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang.

Hubble took a second deep look in the Southern Hemisphere in October of 1998 the HDF-South to see if a similar result would be obtained. Each of the Hubble Deep Fields shows hundreds of galaxies in an area of the sky that is as small as the size of President Roosevelt's eye on a dime held at arm's length.

18. Why was a second Deep Field taken?

The HDF-N covers a very small fraction of the sky. It would have taken 27 million fields and well over 500,000 years to use Hubble's Wide Field and Planetary Camera 2 to survey the entire sky to the depths of the HDF. Instead, astronomers rely on several thin "looking-through-a-soda-straw" views across the cosmos to infer the history of star and galaxy formation.

Taking a second Deep Field helped astronomers to confirm that the HDF-N is representative, and that it is not unusual in some way. The two HDFs are, in fact, consistent with the common assumption of astronomers that the universe should look largely the same in any direction.

19. How were the two Hubble Deep Field sites chosen?

Each of the Hubble Deep Fields represents a "carefully selected random spot on the sky." To allow the Hubble Space Telescope to peer deeply into the sky, astronomers selected a special region of Hubble's orbit where Hubble could view the sky without being blocked by Earth or experiencing interference from the Sun or Moon. The field also had to be far away from the plane of our own galaxy to avoid being cluttered with objects within the Milky Way. Finally, the field needed to have nearby "guide stars," which are used to keep Hubble pointed at the field. These criteria led to the selection of a spot of sky near the handle of the Big Dipper, in the Northern Hemisphere, and a spot of the sky in the constellation Tucana, in the Southern Hemisphere.

20. What is the importance of the HDFs?

When produced, the HDFs contained the faintest galaxies we'd ever been able to see over a large range of distances. Since seemingly "empty" spots were chosen, most of the galaxies in the Deep Fields lie billions of light-years away.

The images show some galaxies in their early stages of formation, appearing in peculiar shapes never previously seen by astronomers. The variety of new galaxy shapes and colors seen in the HDFs, along with the classical elliptical and spiral shapes, are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang. The HDFs are important because they can help answer such questions as:

How many galaxies are there in the universe? The Hubble Deep Field images were used to count galaxies ten times as faint as the deepest existing ground-based optical observations and nearly twice as faint as previous Hubble images.

How did large-scale structure evolve in the universe? The Hubble Deep Field images were used to perform a statistical study of the distribution of galaxies in the sky. This is an essential test of models for the structure of the universe and galaxy formation theories. The Hubble Deep Field images pushed such studies to fainter limits.

How were galaxies assembled? Detailed studies of the ages and chemical compositions of stars in our own galaxy suggest that it has led a relatively quiet existence, forming stars at a rate of a few suns a year for the last 10 billion years. Other spiral galaxies seem to have similar histories.

If this is typical evolution for spiral galaxies, then predictions can be made for what they should have looked like at half their present age their size, color and abundance. This information, combined with actual distances derived from ground-based spectroscopic observations, will provide a new test for theories of spiral galaxies.

The other major class of galaxies seen in the nearby universe is the elliptical type football-shaped aggregates of stars that appear to be very old and stopped forming stars long ago. There is currently debate about when such galaxies formed and whether they formed through collisions of other types of galaxies or through collapse of a pristine cloud of primordial gas in the very early universe.

The Hubble Deep Field images, along with other deep Hubble images, provide a snapshot through time, which can be used to search for distant elliptical galaxies or primeval galaxies that might later evolve into elliptical galaxies.

21. What are some of the key findings learned from the HDF-N image?

In the document "Summary of Key Findings from the Hubble Deep Field" http://oposite.stsci.edu/pubinfo/pr/97/hdf-key-findings.html, you will find information under these headings:

Small Galaxies in the Early Universe

Open versus Closed Universe

Disturbed Galaxies

Stellar Baby Boom

In Search of Hidden Stars

Missing Mass Still Missing

22. It is hard to see the shapes of some of the galaxies in the HDFs. How do astronomers classify them?

They use the colors of the galaxies. Different types of galaxies tend to be different colors. For example, elliptical galaxies have reddish colors because they are mostly composed of old red stars. Astronomers study the colors of nearby elliptical, spiral, and irregular galaxies and compare these colors to those of the galaxies in the Hubble Deep Fields. Comparing the colors allows them to classify the galaxies.

23. If there are thousands of galaxies visible in the Hubble Deep Fields, why do the databases use just over 1000 as the populations of each HDF?

Because of the way astronomers' instruments work, they can be reasonably sure that they have detected all galaxies with a certain range of brightnesses in the Hubble Deep Fields. Astronomers may be able to identify fainter objects, but they cannot be sure that they have detected all of the fainter objects that exist.

When studying populations of objects, astronomers need to make sure that the sample they choose is representative. The very faintest objects do not form a representative sample since astronomers do not know if they have detected all of the faintest objects. Therefore, they limit their sample to objects in a certain brightness range. The sample is then said to be "statistically complete" to that brightness level. For the HDF-N, the statistically complete sample consists of 1067 galaxies.

24. What is the Hubble Ultra Deep Field?

In 2003, the Hubble Space Telescope looked even farther back in time and found as many as 10,000 galaxies, using the recently installed Advanced Camera for Surveys (ACS) and the recently-revived Near Infrared Camera and Multi-object Spectrometer (NICMOS). The million-second-long exposure (11.3 days) captured galaxies that are too faint to be seen by ground-based telescopes, or even by Hubble in its previous faraway looks, called the Hubble Deep Fields.

The ACS image, known as the Hubble Ultra Deep Field (HUDF), shows a wide range of galaxies of various sizes, shapes, colors and ages. Some were formed just a short time after the universe was created, about 13 billion years ago. The infrared NICMOS image complements the visible-light ACS image and reveals some of the most-distant galaxies ever seen.

The ACS picture required a series of exposures taken over the course of 400 Hubble orbits around Earth. This is such a big chunk of the telescope's annual observing time that Space Telescope Science Institute Director, Steven Beckwith, used his own Director's Discretionary Time to provide the needed resources. Just like the previous HDFs, the new data are expected to galvanize the astronomical community and lead to dozens of research papers that will offer new insights into the birth and evolution of galaxies.

25. How does the Hubble Ultra Deep Field compare to the Hubble Deep Fields?

The Hubble Deep Fields represented the faintest, deepest visible light images ever taken using the technology available at that time the Wide Field and Planetary Camera 2. The Hubble Ultra Deep Field was produced using an instrument with newer technology the Advanced Camera for Surveys. The differences in the HDF and HUDF are due to the quality of the instruments being used. Concerning the variety of galaxies visible in the fields, it has been said that the HDF captured images of galaxies when they were youngsters but the HUDF captured images of galaxies as toddlers.

Tales of ...The glorious end of stellar life See image below

Garden-variety stars like our sun live undistinguished lives in their galactic neighborhoods, churning out heat and light for billions of years. When these stars reach retirement age, however, they become unique works of art.

As ordinary, sun-like stars begin their 30,000-year journey into their twilight years, they swell and glow, shrugging off their gaseous layers until only their small, hot cores remain. The ejected gaseous layers are called planetary nebulae, so named in the 18th century because, through small telescopes, these gas clouds had round shapes similar to distant planets such as Uranus or Neptune. (Continued >>)Nebula gallery

A glowing gallery of planetary nebulae

The gaseous debris glows like a fluorescent design, producing objects with striking shapes and names like "Cat's Eye" and "Hourglass." Astronomers have recorded more than 1000 of them in our galaxy.

Gas released by these dying stars helps create new life. This gas contains new chemical elements, including carbon, which eventually are incorporated into stars and planets. Scientists believe that the carbon found on Earth came, in part, from planetary nebulae billions of years ago. The rest was released into space by supernova explosions.

Supernova explosions may be more powerful, but the light show from the death of ordinary stars is more captivating. As bright as 1 billion suns, supernovae explosions signal the demise of massive stars (roughly 8 solar masses or more). These powerful blasts are thought to occur only once every century or so in galaxies like ours. Ordinary stars, on the other hand, die at an average rate of about one per year. By understanding how these garden-variety stars live and die, scientists are developing a clearer picture of our sun's fate. (The Sun will enter its twilight years in another 5 billion years.)

An uneasy peace

Like humans, sun-like stars are born, live their lives, and then die. A sun-like star's life lasts about 10 billion years. Most of that time is spent in adulthood or the "main sequence" phase, living a blissful life in a suburban galaxy neighborhood. A star's peaceful appearance, however, belies what is happening inside its core, where its energy-producing "engine" resides. A highly powerful, self-regulated, 17,000,000 C engine powers the Sun. The engine is constantly converting hydrogen to helium (in a process called nuclear fusion), which produces the energy necessary to sustain life. The Sun's engine produces the heat that makes the Earth habitable. Energy generated by the core also keeps gravity at bay.

All stars wage a continuous battle against gravity specifically, the crushing weight of their outer layers. During most of a star's lifetime, pressure and gravity maintain an uneasy truce. It is like two people arm wrestling to a draw. The weight of a star's outer layers pushes against its inner layers. At the same time, heat generated in its high-metabolism core (by the conversion of hydrogen to helium) produces pressure. This pressure exerts an outward force like the pressure of gas in a hot air balloon to combat the inward force of gravity.

The golden years

As a star ages, it begins to exhaust its supply of hydrogen. When the hydrogen runs out, there is not enough gas pressure inside a star to fight off gravity. A star, then, must make adjustments to keep on running. This signals the beginning of a star's twilight years.

Once the hydrogen in the star's core runs out and gravity begins to claim its victory, the core begins to contract and become denser and hotter. At this point a sun-like star has completed 90 to 95 percent of its lifetime. Its most glorious days are yet to come, however.

As the core continues to contract, the star's energy production increases. To cope with this energy increase, the star swells up to 200 times its normal diameter and becomes about 3000 times more luminous. The result is a red giant star, formed over a period of about 1 billion years.

The battle between gravity and pressure continues in the red giant, forcing the core to contract until it begins to fuse helium into carbon. The resulting energy release allows the core to expand, and the rate of energy production then drops.

A star's final moments

Once the helium is exhausted, the core again becomes inactive. The red giant is dying, but the inactive carbon core is still very hot. Surrounding the core are two shells rich in unprocessed hydrogen and helium.

The star's surface pulsates and shudders with seismic energy from the activity of the shells beneath it. With each pulse, which last about a year, the surface layers expand and cool. Each time this happens some of the stellar exterior is flung into space and carried away in a "slow wind," traveling at about 10 miles per second. This process continues for a few thousand years until only about two-thirds of the star's mass remains: its carbon-oxygen core.

In a few thousand years, as these last outer layers are stripped off, much hotter inner layers of the star become exposed. Soon only the bare carbon-oxygen core is left. The core's temperature is rising rapidly. Over about 20,000 years, the core's surface temperature leaps to approximately 140,000 C, compared with about 6100 C for the surface of a sun-like star. The dense carbon-oxygen star is not much larger than Earth.

Ultraviolet light from this intensely hot surface heads into the star's former outer layers, which are still moving outward in space at 10 miles per second. This light is so energetic that it causes the gas to fluoresce like a fluorescent light bulb forming the bright planetary nebulae surrounding dying stars.

A new wind, which carries very little mass but lots of energy, is blown outward at 1000 miles per second (3.6 million mph). The low-density wind races outward and snowplows into the older gas. This so-called "fast wind" helps to sculpt planetary nebulae, creating some strikingly remarkable shapes.

The star's radiation begins to heat the planetary nebula, causing different gases to glow. The various colors in images of planetary nebulae are associated with these glowing gases. From far away, the former layers of the star appear as a glowing planetary nebula, about 1000 times the size of our solar system. The fluorescent light of planetary nebulae lasts for about 10,000 years.

Eventually, the core stops ejecting gas into space. The dying star is on the path to becoming a slowly fading white dwarf a hot, Earth-sized fossil. The gas expelled earlier ultimately swirls away and merges into the interstellar medium, much as smoke from a train dissipates in our atmosphere. The gas carries traces of newly minted carbon and nitrogen from the atmosphere of the dying star. This material wanders through space until it is drawn into a newly forming star.

Gravity

1. What is gravity?

Gravity is the attraction of every body to every other body due to the masses of each body. The larger the mass, the greater the force. It also depends on the distances: the closer the bodies, the greater the force. Gravity is directed toward the center of a body, and the distance is measured from the center. Gravity keeps the moon going around the Earth, the Earth going around the Sun, and the Sun going around the center of the Milky Way. Gravity is the weakest of the four fundamental forces of nature (electromagnetic, weak nuclear, and strong nuclear are the other three), yet it is the force that governs motion in the universe.

2. What factors determine the force of gravity between two bodies?

The force of gravity (F) depends on the masses of the two bodies (m1, m2) and the distance between the bodies' centers (r). There is a direct proportion between mass and gravitational force: If you double the mass of one body, the gravitational force between them is also doubled. The gravitational force is inversely proportional to the square of the distance: If you double the distance between the two bodies, the force of gravity is reduced to one-fourth its original value. The equation relating these ideas is: F = G(m1m2)/r2, where G is the universal gravitational constant equal to 6.67 x 10-11 Nm2/kg2 or m3/s2kg).

3. What keeps objects in orbit? (What keeps the moon from falling to Earth or the Earth from falling into the Sun?

Objects remain in orbit around a massive body due to gravity and their sideways motion. Objects in orbit are moving sideways, approximately at right angles (90 degrees) to the force of gravity. An object would travel in a straight line with a constant speed if it were not for the gravitational attraction of a massive body. The attractive force changes the motion of the object from a straight line to a closed curve, as it begins orbiting the massive body. In effect, the object is "falling" around the massive body.

4. What is an unbalanced force?

An unbalanced or net force causes changes in an object's speed and/or direction. Only one force acting on a body is unbalanced because there is no counter-force to cancel the force's effects. A person remains at rest when sitting in a chair because there are two balanced forces acting on that person. One of those forces is gravity, which pulls the person downward. The other force is the chair pushing upward on the person. The two forces are equal in magnitude (size) and opposite in direction. So they balance each other, and the person doesn't change direction or speed.

On the other hand, Earth's gravitational influence on the moon is unbalanced. The moon is constantly changing its direction of motion, so it is experiencing acceleration. Any time there is an unbalanced force, the object will undergo a change in direction or speed. So, a change in direction or speed means there is an unbalanced force at work.

5. How does a large body (like a planet) capture a small body (like a comet)?

According to Newton's First Law of Motion, an object in motion tends to remain in straight-line motion at a constant speed unless acted upon by an external, unbalanced force. When a comet or asteroid comes close to a body with a large gravitational force (a planet, for example), the path of the comet or asteroid is altered due to the unbalanced force of gravity on the body. It moves toward the planet as described in Newton's Second Law: When an unbalanced force acts on a body, the body experiences acceleration in the direction of the force. A force that tends to make a body move in a curved path is called a centripetal force.

Occasionally, the comet will be close enough to the planet to become trapped by the gravitational force and will begin orbiting the planet. The comet would like to continue traveling in a straight line, but the planet is pulling the smaller body toward its center, making it travel in a curved path around the planet.

6. Why does the angle of approach affect the ability of a planet to capture a comet?

The more direct the approach, the easier it is for the planet to capture a comet because the comet comes closer to the planet. As discussed in question 5 above, the closer the objects come to each other, the stronger the force of gravity.

7. What role does speed play in the capture of a comet by a planet?

The faster an object is moving, the greater the kinetic energy. In order for an object to be trapped by the gravity of a planet, the object's kinetic energy (Ek = 1/2 mv2 where Ek = kinetic energy, m = mass of comet, and v = speed) must be less than the gravitational potential energy (U = GMm/r where U = potential energy, G = gravitational constant, M = mass of planet, m = mass of comet, and r = distance from the center of the planet to the center of the comet).

The comet's total energy is equal to the kinetic plus potential energies. But the potential energy is negative, so a comet can only escape a planet's gravitational pull if the kinetic energy is larger than the gravitational potential energy.

8. Does a comet's mass play a role in its capture by a planet? Or, why doesn't a comet's mass affect the path it follows?

When a comet travels near a planet, there is a gravitational force between the comet and the planet (Fg = GMm/r2 where Fg = gravitational force and the others are as defined above). This force provides a centripetal acceleration, which changes the comet's path so that it begins orbiting the planet. (Fc= mac where Fc = centripetal force and ac = centripetal acceleration). These two forces are the same. If we set them equal to each other, the mass of the comet factors out of the equation.

Fc = Fgmac = GMm/r2ac = GM/r2

9. How does the mass of a planet affect its ability to capture a comet?

As shown in the equation in question 8 above, the amount of centripetal acceleration on a comet depends on the mass of the body causing the acceleration. The greater the acceleration, the more easily the comet's path is changed and the more likely it is to be captured. This means that a massive planet can capture a comet more easily than could a less massive planet.

10. What causes a comet to break up, and what are tidal forces?

The force that makes a comet orbit a planet is also responsible for the breakup of a comet. That force is gravity. Because the gravitational force increases as the distance between the bodies decreases, the force of gravity on the nearer side of a celestial body is stronger than the force of gravity on the far side, and a tidal force arises.

These forces can exist between any two celestial objects in orbit around each other. Some celestial bodies are not perfectly rigid, so they become distorted when subjected to such tidal forces. It is as if they are being pushed from the top and bottom, and a bulge forms on either side of the body one directed toward the central body and the other on the opposite side. But there isn't a force above and below the body. What is happening is that the part of the orbiting body closest to the central body moves toward that body by a larger amount than the middle of the orbiting body. This causes a bulge on the side toward the central body.

To explain the bulge on the opposite side, apply the same logic: the middle of the orbiting body feels a greater pull than the far side, so it moves toward the central body more than the outer part. This leaves a bulge of material behind. If a celestial body is very rigid or is not held together well, instead of getting pulled out of shape, the tidal forces can actually tear the body apart. This is what happened with comet Shoemaker-Levy 9.

11. Why did Shoemaker-Levy 9 crash into Jupiter?

Comets usually orbit the Sun, but Shoemaker-Levy 9 was captured by Jupiter's gravity and appears to have orbited the planet for about two decades before the breakup. After Shoemaker-Levy 9 broke into fragments, it was in an orbit around Jupiter that had a period of two years. The energy lost in the breakup of the comet lowered the point of closest approach ("perijove") of the subsequent orbit to within one Jupiter radius of that planet's center.