what’s up?? an introduction to the creatures of the night sky begin
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An introduction to the creatures of the night sky
There are some far out things that even the ancients knew about. These include the sun, moon, planets, stars, and comets.
More recently, other objects
have come into view.
The Sun is our star.
It Contains 99% of
the mass of the solar
reactions in the Sun’s
core release the light
and energy needed for life on Earth. The Sun is a 4.6 billion year old star of average size. Learn more
orbits around the Earthat a distance of 240,000 miles. It is smaller than the Earth with a diameter aboutthe same as the distance from Boston to Seattle. The moon’s surface is
covered by craters, darker regions called maria, and lighter regions called terrae. Learn more
Five of the solar system’s nine planets are visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Earth, of course, is under our feet. Planets look different from stars. They’re brighter, they don’t twinkle, and they change location on the stellar background. Each planet is an exciting world in itself. Learn more
Stars are huge balls of ionized gas (plasma). They are 75% hydrogen and 25% helium. Population II stars are younger and contain significant quantities of other elements. Population I stars are older and do not. Stars vary greatly in size, temperature and brightness (luminosity). Most fit into one of four categories: supergiants, red giants, main-sequence stars, and white dwarfs. They shine because they’ve been heated by nuclear fusion reactions in their cores. Learn more
CometsComets are dirty snowballs that fly through the solar system. As they near the sun, heat and charged particles push material off their cores forming beautiful tails. Comets provide hints of what our solar system looked like at its birth. Learn more
Things get a bit Fuzzy• Galileo observed the sky with a telescope for the first
time in 1609. He saw lots and lots more stars. Some stars looked fuzzy. He wondered why.
• In the 1700’s, a fuzzy patch of sky was given the name nebula (plural nebulae). It comes from a Latin word for cloudy.
• In 1758, comet-hunter Charles Messier observed a comet that didn’t move. He called it M1. He observed many other similar objects. His catalogue eventually included 110 “Messier Objects.”
• In 1864, The General Cattalogue of Nebulae was published with 5,097 entries. Most of these were discovered by William Herschel and his son John.
These fuzzy patches turned out to be:
• Clusters – stars grouped close together within our own galaxy, learn more
• Galaxies – collections of billions of stars very far away, learn more
• Nebulae – clouds of gas between stars, learn more
• Other strange stuff – things for you to research
Clusters – stars grouped close together within our
There are two different types of clusters
contain between 100,000 and 1,000,000 gravitationally bound older stars. About 200 of them are evenly distributed about the halo of our galaxy. They are 7-120 pc in diameter and can be round or slightly elliptical. They’ve helped us understand the rotation of the galaxy and the evolution of stars.
are smaller groups of gravitationally bound stars. These stars are generally younger and metal-rich. Open clusters orbit around the center of our galaxy. They provide important clues about stellar evolution and the behavior of the Milky Way.
Galaxies – collections of billions of stars very far
awayThere are three types of galaxies, each classified by its appearance. Our galaxy,
the Milk Way, is a spiral galaxy.
Spiral GalaxiesAndromeda Galaxy
M 51 (Whirlpool Galaxy)
- Small Magellanic Cloud
Nebulae – clouds of gas between stars
The space between stars is not empty. It is filled with the Interstellar Medium (ISM).
More dense clouds of gas can be found in:Corona (Circumstellar Gas)
Emission Nebula (HII regions)Reflection Nebula (diffuse nebula)
Planetary NebulaSupernova Remnants (SNR)
Dark Nebulae (Bok Globules) Giant Molecular Clouds (GMC)
Interstellar Medium (ISM)
On average, there is a 2-3 pc space between stars in the Milky Way. About 9% of the known matter in our galaxy exists in the ISM. The ISM can be observed either as it reflects light from nearby stars or as it absorbs their light. Hydrogen accounts for 90% of the matter in the ISM, sometimes in molecular clouds as big as 1,000 solar masses. These clouds can be cold and dense. They account for ½ the total mass of the ISM. Most of the gas in the ISM is located on the Milky Way’s 4 spiral arms. This gas is mostly charged. It is pushed out of our solar system by the solar wind although some neutral particles make it through to Earth. Outside of its clouds, the ISM is very dilute and very hot. It is populated by molecules and grains of dust.
Dust is made of very small (.01 to .001 micrometer) needlelike silicates of Al, Fe, Mg, and graphite. Water, methane, or ammonia is then frozen on, forming an outer layer. Although it accounts for only 1-2% off all the known matter in the galaxy, interstellar dust has a profound effect on the light we receive from the sky. It absorbs up to half the emitted starlight and reemits it in the infrared. It provides a site for the bonding of the hydrogen molecule, the most common molecule in the universe. It linearly (and sometimes circularly) polarizes incoming light. It is used by scientists to measure the magnetic field in space. Dust is very important to the chemistry of space.
Corona (Circumstellar Gas)
is very hot (1,000,000 K) sparse gas emitted from the surface of stars. It is ionized by collisions. Some of the metals in this gas have lost several electrons.
our sun’s corona
Emission Nebulae (HII regions)
are heated with UV radiation by hot, young, bright stars. This radiation is energetic enough to ionize the hydrogen in these clouds. The nebulae then glow red. They are hot, 10,000K, and are active sights for star formation.
M42 (Orion Nebula)
Reflection Nebulaeare also called diffuse nebula. They are Illuminated by white stars, but these clouds of gas generally glow blue because blue light is more easily scattered.
Not where planets are formed!! Planetary Nebula form around small (1-5 solar mass) stars between their red giant and white dwarf stages. They form as the red giant loses its mass to the stellar wind and are therefore generally rich in chemicals. Our sun will generate a planetary nebula in about 5 billion years. They are very small, typically only about one light year across, and spherically shaped. Their outer regions have expanded and cooled, leaving them redder than the center.
M 57 (Ring Nebula)
M27 (Dumbell Nebula)
Supernova Remnants (SNR)
are hot regions only a couple light years across. They were formed as a supernova’s shock wave rippled through space collecting and heating all the matter in its way. They are hot, slowly cooling regions.
M1 (Crab Nebula)
Dark Nebulae (Bok Globules)
are dark spots on the night sky. They are gravitationally bound clouds of dust. These regions are usually about 1 pc across and about 200 solar masses. They can lead to star formation.
Giant Molecular Clouds (GMC)
With a mass of 100,000 solar masses and a diameter of 100pc, they are the biggest things in the Galaxy. Like nebulae, they are more commonly found close to the galactic center. Some have hot cores. When one star is formed in a Giant Molecular Cloud, it can produce a shock wave that leads to the formation of other stars and other shock waves.
Other Strange Stuff
In the 20th century, scientists started looking at the sky in different parts of the
electromagnetic spectrum. They saw even stranger objects like:
quasars, blazars, neutron stars, white dwarfs, black holes, pulsars, masers
, variable stars, extra-solar planets, etc.