Stars and Galaxies

Important Vocabulary

Absolute Magnitude• How bright a star would

look if it were a standard position from Earth.

Apparent Magnitude• How bright a star looks

from Earth

Astronomical Unit-The distance from the Earth to the Sun. Usually written AU = about 93 million miles.

Light YearThe distance which a ray of light would travel in one year. This is about 6,000,000,000,000 (6 trillion) miles. Light travels at a speed of 186,000 miles per second

What is a star?

• Stars are balls of gases that release heat energy produced by nuclear reaction within their cores. Throughout the galaxy, stars are constantly forming, evolving, and being destroyed.

Why study stars?

• Most stars are similar to our nearest star, the sun.

• By studying a range of stars, astronomers can figure out how stars changed over time and hence understand more about the past and probable future of our sun.

Get to know our closest star…the sun!

• It is a ball of glowing gas of average size, temperature, and brightness when compared to other stars

• The sun’s surface is known as the photosphere surrounded by a vapory layer called the chromosphere

• Sunspots are relatively cool areas on the sun

More about our sun

• The sun is located approximately 93,000,000 million miles (1 AU) from Earth.

• It is estimated to reach about 27 million degrees Fahrenheit (15 million degrees Celsius)

• In a nuclear reaction, hydrogen is converted to helium by fusion

How is a star born?• Stars form in huge clouds of dust and gas

called nebula(e).• The nebula(e) shrinks under the inward pull of

its own gravity then becoming a premature baby star known as a protostar.

A star is born…

• The protostar continues to build in temperature and density switching itself on generating its own heat and light.

Main Sequence Star

This is the star’s main lifetime…billions of years are spent in this stage…our sun is in main sequence right now. Most stars (about 90%) are Main Sequence Stars.

For these stars, the hotter they are, the brighter they are.

Main Sequence Stars can grow, then die!

Massive stars• Mass greater than 10 of our suns

(600 million miles across)• Relatively old stars

• Continues to grow to a red supergiant expanding its outer layers.

• Betelgeuse and Rigel are super giants. These stars are rare.

• Eventually its core collapses causing a huge explosion known as a supernova

Weeks after- can continue to glow brightly but as parts are scattered over universe from explosion, core can be crushed into tiny super dense neutron star possibly turning into a black hole

Small/Medium Stars

• Mass about 60 million miles across• Swells into a red giant, eventually losing its outer

layers• Forms into a gas shell called planetary nebula(e)• Continues to cool and fade so that core is exposed as

a white dwarf for billions of years• Core glows red (like an ember) as continues to cool• Core stops glowing then known as a black dwarf (none known yet; universe too young)

Main Sequence Stars(Young Stars)

Yellow Dwarf• Relatively small • Can be up to 20 times larger

than our sun and up to 20,000 times brighter

• Our sun is one

Red Dwarf• A small, cool, very faint star

whose surface temperature is under 4,000 K.

• Most common type of star• Proxima Centauri is one

More Star TypesWhite Dwarf

• Ancient white dwarf stars shine in the Milky Way galaxy. White dwarfs are stars that have burned up all of the hydrogen and helium they once used as nuclear fuel to elements such as carbon, nitrogen, and oxygen

Black Dwarf

• A 'black dwarf' is a white dwarf that has cooled down enough that it no longer emits light.

More Star Types

Red Giant• Towards the end of a star's life, the

temperature near the core rises and this causes the size of the star to expand. This is the fate of the Sun in about 5 billion years.

• Stars convert hydrogen to helium to produce light (and other radiation). As time progresses, the heavier helium sinks to the center of the star, with a shell of hydrogen around this helium center core. The hydrogen is depleted so it no longer generates enough energy and pressure to support the outer layers of the star. As the star collapses, the pressure and temperature rises. Then , the star expands into a Red Giant.

Blue Giant

• Because blue stars are large, and compact, they burn their fuel quickly, which gives them a very high temperature. These stars often run out of fuel in only 10,000 - 100,000 years.

• A blue giant is very bright. Like a light house, they shine across a great distance. Even though blue giant stars are rare, they make up many of the stars we see at night.

• Blue giant stars die in a spectacular way. They grow larger just like the Sun sized stars, but then instead of shrinking and forming a planetary nebula they explode in what is called a super nova. Super nova explosions can be brighter than an entire galaxy, and can be seen from very far away.

More Star Types

Nova • A star which suddenly

flares up to many times its original brightness before fading again.

Supernova Is a super bright explosion

of a star. A supernova can produce the same amount of energy in one second, as an entire galaxy.

Death of a Star

Explanation: What would you see if you went right up to a black hole? Above are two computer generated images highlighting how strange things would look. On the left is a normal star field containing the constellation Orion. Notice the three stars of nearly equal brightness that make up Orion's Belt. On the right is the same star field but this time with a black hole superposed in the center of the frame. The black hole has such strong gravity that light is noticeably bent towards it. Black holes may eject a great amount of radioactive energy known as a quasar.

More information on star types

Neutron Star• Neutron stars are created in the cores of

massive stars during supernova explosions. • The core of the star collapses, and crushes

together every proton with a matching electron turning them into a neutron. The neutrons, however, can often stop the collapse and remain as a neutron star.

• They are the most dense objects known. They are only about 10 miles in diameter, yet they are more massive than the Sun. One sugar cube size of a neutron star weighs about 100 million tons, which is about as much as a mountain.

• Like their less massive counterparts, white dwarfs, the heavier a neutron star gets the smaller it gets. Imagine if a 10 pound bag of flour was smaller than a 5 pound bag!

Spinning neutron stars that emit radio waves are called pulsars.

Stars Life Cycle

Nebulae →Proto star→ Main Sequence Star ↙ ↘ If … massive star if …smaller or medium star Red Giant or Blue Giant White Dwarf →Black Dwarf ↙ ↘ • Nova or Supernova• ↓ ↘• Neutron Stars or Black Hole

The End Of Part One Stars

Constellations• The constellations have been called humanity's oldest picture book. For

5,000 and more years, people have looked into a clear night sky and seen the same stars we see today. They isolated groups of stars and connected them to each other with imaginary lines, much as we play connect-the-dots.

• In the past, people had an excellent knowledge of the night sky. They were able to tell when to plant and when to harvest, and later they navigated the seas with the stars' help. Characters of myth and legend were used to name and tell the stories of the stars such as the group of stars that looked like a man with a sword (or bow and arrow) was named Orion, for the famous hunter in Greek mythology.

Stars can form patterns in the sky called constellations

How are they helpful?

• The two brightest stars in the Big Dipper "point" to the North Star. The North Star, which is also called Polaris, is a star that always points to true north, while the other stars in the northern sky seem to circle around it.

• Because of its seemingly fixed position in the sky, sailors and travelers have been able to use the north star to guide them on their travels.

Finding the North StarFinding the Big Dipper is Key to Finding the North Star

The key to locating the North Star in the night sky is to first find the Big Dipper, a constellation of stars known as Ursa Major.. The Big Dipper is perhaps the best known group of stars in the northern sky and is easy to distinguish from all others. Also known as the Great Bear, the Big Dipper is located just north of the celestial pole. Knowing how to find the Big Dipper makes it easy to find the north star.

The second key to finding the North Star is a similarly shaped constellation of stars known as the Little Dipper. The Little Dipper, also known as Ursa Minor, is smaller and more difficult to find in the night sky. Fortunately its big brother, the Big Dipper points the way.

The Pole Star we are seeking is the brightest of the Little Dipper stars and is located at the tip of the dipper’s handle.

Diagram of Big and Little Dippers

Locating the North Star is easy if you follow this simple diagram.

1. Find the Big Dipper.  Draw an straight line between the two stars of the Big Dipper as shown, toward the Little Dipper.2. The North Star, the brightest star in this constellation, is located at the end of the handle of the Little Dipper.

ZodiacTwelve constellations, together called the Zodiac, form a belt around the earth. As the earth

revolves around the sun, a different part of the sky becomes visible until, after a year, the earth has completed one trip and starts again. Each month, one of the 12 constellations appears above

the horizon in the east to begin its march across the sky. Night after night, the constellation appears to move across our sky until it disappears below the horizon in the west and the next

constellation appears in the east.

The 12 Constellations of the Zodiac

Galaxies

Our stars and planets are all part of a galaxy

• Galaxies are formed by massive star clumps. This process takes place anywhere from 0.5 to 1 billion years. Usually stars are in a system of two or more stars which are clumped together.

• After about 1-2 billion years, the clumps grow large enough to be seen by the Hubble Space Telescope. After 2-4 billion years, collisions and mergers occur, making the formation of larger, irregular-looking objects

• After 4 -13 billion years, galaxies form and take their final shapes. We can see these galaxies today.

Sometimes Galaxies can combine(like Andromeda and the Milky Way are predicted to do someday)

Before

AFTER

How do galaxies get their shapes?• Galaxies form out of gigantic gas clouds that develop in early space. These

clouds collapse due to the forces of gravity. Different properties of the collapse create different shapes of galaxies.

Our galaxy is the Milky Way: it is a barred spiral galaxy

Andromeda is the galaxy next door. It is a spiral galaxy.

Ceres is an elliptical galaxy

A galaxy without a particular shape is known as an irregular galaxy

THE END or is it?

• As technology continues to grow more and more advanced we’re are sure to learn more and more about stars and the galaxies in which they belong.

• REMEMBER: We don’t see galaxies or stars the way they are. We see them the way they were!