STARS & GALAXIES

The View From Earth

Apparent Motion

Because of rotation of Earth on axis, stars seem to move across night sky, but it’s actually just the Earth’s movement

Notice they seem to move around 1 center point?

Polaris – star directly above North Pole

Earth rotates around this point, so Polaris does not appear to move

Stars located on the side of the sun opposite Earth are blocked by the sun

Different stars are visible during different seasons, depending on where Earth is in relation to the sun

Constellations Constellation pattern of stars

Astronomers recognize 88 constellations

Some named for real or imaginary animals Ex. Ursa Major – the great bear

Draco – the dragon

Others named for ancient gods or legendary heroes

Hercules Orion, the Hunter

• Astronomers divided sky into sections using constellations

• Can use like map to locate specific star

Seeing Stars

Astronomers (scientists who study stars and space) use telescopes on Earth.

They also use the Hubble Space Telescope

Telescopes

Telescopes can collect much more light than the human eye can detect.

Interference from atmosphere makes viewing stars difficult

Visible light is just one part of the Electromagnetic Spectrum

The Electromagnetic Spectrum

The different parts of the EM Spectrum have different wavelengths.

Longer wavelengths have low energy Shorter wavelengths have high energy

Electromagnetic spectrum

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/what_is_ir.html

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Spectroscopes All stars emit radiation

Radio, infrared, visible, ultraviolet, x-ray and even some gamma rays The range of wavelengths that a star emits is the star’s Spectrum

(plural, spectra)

Scientists analyze the spectra of stars(starlight) using a spectroscope separates light into different wavelengths

By analyzing a star’s spectrum, astronomers can learn about it’s composition and temperature.

Every element has characteristic spectrum

Colors and lines in spectrum of star tell which elements star is made of

A star’s light has dark bands along the spectrum, these bands are caused by the absorption of certain wavelengths of light by specific gases in the star.

Different bands show what elements are in the star’s atmosphere.

Hydrogen and helium make up most stars

Measuring Distances

One way astronomers use to determine distances is parallax- the apparent change in an object’s position caused by looking at it from two different points.

Example of parallax Hold out your arm in front of you with your thumb up Close one eye and note the position of your thumb

against the background Open that eye and close the other one Does your thumb shift? The closer your thumb to your face, the greater the

shift

As earth circles the sun, observers study stars from slightly different angles

During 6-month period, closer star shifts relative to stars farther away

Closer star, more shift

Measuring Distance Astronomical Units (AU)- the average

distance between the Earth and the Sun. 1AU= approximately 150 million km. It is used to measure distances within the solar

system.

Astronomers use light years to measure distance outside of the solar system. light year distance light travels in one year Speed of light = 300,000 km/s Light travels about 9.5 trillion km in 1 year Light from sun takes 8 minutes to reach Earth

Closest star in this system is Proxima Centauri (4.2-light years away)

Brightest star seen from Earth is Sirius ( 9 light years-away)

Polaris (North Star) is 700 light-years away

Measuring Brightness

Scientists measure a star’s brightness in two ways: Apparent Magnitude-How bright the star

seems to be when viewed from Earth (depends on distance from Earth)

Absolute Magnitude-How bright the star actually is.

From these measurements, astronomers can calculate a star’s distance from Earth

Stars appear to be bright or dim depending on their distance from Earth, but stars have an actual , or absolute magnitude.

The true brightness of an object is called it’s luminosity.

A star’s luminosity depends on the star’s temperature and size, but not it’s distance from Earth.

The Sun & Other Stars

Nuclear Fusion- a process that occurs when the nuclei of several atoms combine into one large nucleus. This releases a great amount of energy This energy powers stars.

Star a large ball of gas held together by gravity with a core so hot nuclear fusion occurs.

The Inner Layers of Our Sun

1. Core – contains hydrogen (H) and helium (He); where nuclear fusion occurs (H atoms smash together to form He atoms); fusion releases lots of electromagnetic energy; temperature is 15 million Kelvin

2. Radiative Zone – made of cooler, denser H; light energy is released from this zone

3. Convection Zone – contains convection currents where hot gas moves to the surface and cooler gas sinks to the interior

The Outer Layers of Our Sun

4. Photosphere – surface of the star; dense, bright part that we see; looks smooth but made of gas; temperature 5800 K

5. Chromosphere – orange-red layer above the photosphere

6. Corona – wide, outermost layer of star’s atmosphere; has an irregular shape

Other Features of Our SunSunspots – dark splotches that are cooler; regions of strong magnetic activity; peak every 11 years

Prominences and Flares – • prominences -clouds of gas that make loops and jets into the

corona; last for weeks

• flares -sudden increases in brightness near sunspots or prominences; violent eruptions that last for hours

Other Features of Our Sun

http://solarscience.msfc.nasa.gov/images/combo.gifhttp://www.nasa.gov/topics/solarsystem/features/dream-cme.html

http://www.geog.ucsb.edu/~jeff/wallpaper2/page3.html

Coronal Mass Ejections – huge bubbles of gas are ejected from the corona; occur over hours and can reach Earth; cause radio blackouts and satellite malfunctions

Solar Wind – charged particles that stream from the Sun, pass Earth, and reach the edge of the Solar System; auroras are created when charged particles in the solar wind interact with Earth’s magnetic field

http://www.destination360.com/north-america/us/alaska/aurora-borealis

Groups of Stars

Some stars are single stars that have no stellar companion. (ex: our sun)

The most common star system is a binary system, where two stars orbit each other.

By studying the orbits of binary stars, astronomers can determine the stars’ masses.

Classification of Stars Scientists use a star’s spectrum-the light

it emits out by wave length and the star’s color to classify a star. A star’s color is usually related to the

star’s temperature. Red stars are the coolest stars blue-white stars are the hottest.

A star’s color is also closely related to its mass; Blue-white stars have the most mass Red stars have the least mass.

Color Surface Temperature (˚C)

Blue Above 30,000

Blue-white 10,000-30,000

Blue-white 7,500-10,000

Yellow-white 6,000-7,500

Yellow 5,000-6,000

Orange 3,500-5,000

Red Less than 3,500

H-R Diagram

The Hartzsprung-Russell diagram shows increasing luminosity of stars on the y-axis and decreasing temperature of stars on the x-axis.

Brightness increases as surface temperature increases

Most stars fall in band running through the middle

Main-sequence stars

Our sun is a main sequence star according to the H-R Diagram.

The actual brightness is average for a star of its average size.

Upper right corner are cool, bright stars giants

Some are so big they are called supergiants

Lower left of H-R diagram are hot but dim (very small)

White dwarf usually about the size of Earth

STELLAR EVOLUTION

• Typical star exists for billions of years.

Astronomers never able to observe one star through its whole life

Instead, they develop theories about evolution of stars by studying stars in different stages

THE ORIGIN OF STARS

• Star begins in a nebula cloud of gas and dust

• Gravity causes the densest parts of a star- forming nebula to collapse, forming a region called a protostar.

• Gravity pulls more material toward center of protostar• Pressure ↑, heat ↑

A developing Protostar gets increasingly hotter over many thousands of years, heating up the surrounding gas and dust.

The heated gas and dust eventually blow away, and the protostar becomes a visible star.

The gas and dust might later become planets or other objects that orbit the star.

Main-Sequence Stars Second and longest

stage in life of star

Energy made in core of star as H atoms fuse into He atoms

A star leaves the main sequence when its supply of Hydrogen has been nearly used up.

Giants and Supergiants

3rd stage – when almost all H is fused to He

Without H as fuel, star contracts under gravity

Contraction increases temperature in core

Combine H fusion (in outer shell) and He fusion (in core) releases energy

Causes outer shell to expand greatly

Outer shell of gases cools

Becomes red giant/red supergiant

Red giant 10+ times bigger than sun

Red supergiant 100+ times bigger than sun

Stages in life of star cover ENORMOUS periods of time

Scientists estimate over 5 BILLION YEARS, the sun (main-sequence) has only fused 5% of its hydrogen

END OF A STAR

All stars form in the same way, but stars die in different ways

White Dwarf Stars Lower mass stars such as the Sun do not have

enough mass to fuse elements heavier than helium.

Stars with a mass less than 10 times the mass of the Sun will eventually become a white dwarf.

End of giant stage is the end of helium fusion Energy no longer available Star loses outer gases Exposes a core Becomes a planetary nebula

Gravity causes last matter to be pulled in

What’s left hot, dense core of matter (white dwarf)

Shine for billions of years before cooling completely

When ALL energy gone black dwarf

Don’t exist yet, universe not old enough

Supernova Stars with masses 10-100 times

greater than sun become supernovas stars that have such large explosion that it blows itself apart

1054 – Chinese saw explosion so bright it was seen during the day for 3 weeks

Neutron Stars

After explosion, supernova may contract into small but DENSE ball of neutrons neutron star

Spoonful of neutron star would weigh 100 million tons on earth

Rotate quickly Diameter about 30 km

Pulsars

Some neutron stars release 2 beams of radiation

Sweep across universe like lighthouse

Called pulsars Radiation detected as radio waves

Black Holes

Some massive stars make extras too massive to become neutron stars

Contraction crushes dense core Leaves hole in space black hole Gravity so great not even light can

escape

Black holes don’t give off light so locating them is difficult

Find by effects on nearby stars

Matter from nearby star pulled into black hole

Just before it goes in, X rays are released

Recycling Matter The gas that stars give off at the end of their

life cycle gets recycled; it is the material that forms new stars and planets. A white dwarf casts off hydrogen and helium,

which becomes part of a planetary nebula; these gases form new stars, not new planets

A supernova produces a shock wave that pushes on the gas and dust in space Almost all the elements that are heavier than hydrogen and

helium, including carbon, silicon, and oxygen, were released into the universe by supernovae.

The force of gravity causes matter in nebulae to clump together and eventually form new stars and planets.

GALAXIES & THE UNIVERSE

Galaxies

Galaxy large-scale group of stars

Held together by gravity Most matter in galaxies

is dark matter, which emits no light at any wavelength

90% of Universe’s mass is thought to be dark matter

Types of Galaxies Classified into 3 main

types1. Spiral

has nucleus (center) of bright stars

Flat arms that spiral around center

Arms have millions of young stars, gas, dust

2. Elliptical Galaxies

Very bright in center No spiral arms Have no young stars Contain very little gas and

dust

3. Irregular Galaxies No particular shape Usually smaller and

fainter.

Gravity pulls galaxies together in groups called clusters

Clusters clump into large groups called superclusters.

Regions of empty space between the superclusters cause the universe to have structure like a sponge.

The Milky Way Our solar system is

in the Milky Way Galaxy

It is a spiral galaxy that contains, dust, and almost 200 billion stars

Our solar system is located in a spiral arm of the Milky Way.

The Big Bang Theory

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By looking into space, astronomers can look back in time.

The Big Bang Theory 13-14 billion years ago,

“big bang” happened

Universe started expanding

Still moving outward

Evidence of Big Bang Spectrum of star moving

toward or away from Earth appears to shift

Doppler effect apparent shift in wavelength of light produced by light source moving toward or away from observer

Moving toward the Earth – wavelength decreases, more toward blue end of spectrum (blue shift)

Moving away from Earth – wavelength increases, more toward red end of spectrum (red shift)

Most distant galaxies have red-shifted spectra

Galaxies are moving away from Earth Scientists are researching dark energy that could be pushing

galaxies together.

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