StarsUnit 6 --- Sixth Gradehttp://www.youtube.com/watch?v=QZDk1cbKp7s&feature=related&safety_mode=true&persist_safety_mode=1

Most stars look like faint dots of light in the night sky…

But they are actually huge, hot bright balls of gas that are trillions of kilometers away from Earth

The Sun is our closest star, it’s 93 million miles away, and it takes about 8 minutes for its light to reach us on Earth.

The next closest star

The next closest star is Proxima Centauri, located 4.2 light years away.

A light year is the distance light travels in a single year – 9,460,528,000,000 kilometers, or 5,865,696,000,000 miles

▪ Approximately 6 trillion

The light we see from Promima Centauri left the star 4.2 years ago

It would take our fastest spacecraft 50,000 years to reach Proxima Centauri!

Composition of Stars

Stars are made of different elements in the form of gases. On average, stars are

70 percent hydrogen and 28 percent helium

Our Sun is 91% hydrogen and 9% helium

The inner layers are very dense and hot

The outer layers are made of cooler gases

Determining Composition

Because different elements absorb different wavelengths of light, astronomers can tell what a star is made of from the light they observe from the star A Spectragraph breaks

down a star’s light into a spectrum▪ From the colors on the

spectrum, astronomers can determine what a star is made up of

Classifying Stars

In the 1800’s, aided by their spectragraphs, astronomers started to collect and classify the spectra of many stars.

At first, they were classified according to their composition, but that system was found to be flawed, and we now classify stars according to how hot they are.

Types of StarsClass Color Surface

tempElements Examples

O Blue Above 30,ooo C

helium 1o Lacertae

B Blue-white 10,000-30,000C

Helium and hydrogen

Rigel, Spica

ABlue-white 7,500-

10,000 Chydrogen Vega, Sirius

F Yellow-white 6,000-7,500 C

Hydrogen, heavier elements

Canopus, Procyon

G Yellow 5,000-6,000 C

Calcium, other metals

The Sun, Capella

K orange 3,500- 5,000 C

Calcium, molecules

Arcturus, Aldebaran

M red Less than 3,500 C

molecules Betelgeuse, Antares

Differences in Brightness Stars are further

classified by their brightness, or how brightly they shine in the sky.

At first, they were given numbers to indicate their brightness The brightest stars

were called first-magnitude stars

The dimmest were sixth-degree magnitude stars

Differences in Brightness

But when astronomers began to use telescopes, they began to see many stars they hadn’t discovered because they were so dim.

They added to their scale of magnitudes Bright stars had a negative number Dim stars had a positive number

So How Bright is That Star? Look at the picture ---

Do all the lights look the same?

Do some appear to be brighter or dimmer?

Why? The ones closer appear to

be brighter, and the ones further away appear to be dimmer.

The same thing applies with stars!

Apparent Magnitude

The apparent brightness of a star from Earth is its apparent magnitude.

Apparent magnitude depends on how close the star is to the Earth Closer stars appear to be

brighter▪ The Sun’s apparent

magnitude is -26.8,which means it is the brightest object in the sky (if you are on Earth)

Stars that are farther away appear to be dimmer.

Absolute Magnitude

Absolute Magnitude is the actual brightness of a star.

Astronomers "pretend" to line up stars exactly 10 parsecs (about 32.6 light years) away from Earth.

They then figure out how bright each star would look. They call that brightness the

star's absolute magnitude.▪ In all actuality, the Sun is not

an especially bright star. ▪ The Sun has an absolute

magnitude of 4.83.

Measuring the Distances to Stars As mentioned earlier, the unit of measurement used to

measure the distances to stars is the light-year.

It is not a measurement of time as the name implies, it’s a measurement of distance.

One light year is equal to 9,460,528,000,000 kilometers, or 5,865,696,000,000 miles (6 trillion)

Since it would be very hard to run a measuring tape up into space for millions, billions and trillions of miles, astronomers use other methods to measure a star’s distance from Earth

Parallax

Some stars, the ones that are closest to Earth, appear to move, while those that are far away appear to stay in one place.

This apparent shift is called parallax

Astronomers use parallax and math to find the actual distances to stars that are close to Earth