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Types of Stars
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     - Temperature
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Temperature and Color

Let's find out what causes the valleys in a star's spectrum.

Click on animation to play

When you turn on a burner on a stove, it heats up and starts to glow. The burner gives off light due to its high temperature. It gives off most of its visible light in the red part of the spectrum, so you see it glowing red. (A stove burner actually gives off more light in the infrared. Put your hand above the burner a few inches, and you can feel the infrared radiation on your hand in the form of heat). As you heat up the burner, it glows brighter, and the color changes: first to orange, then yellow, then blue. The animation on the right shows how the colors change as a stove heats up, then cools off again.

Stars are much hotter than stoves, but they also give off light because of their temperatures. Just like stoves, stars give off most of their light in a certain color depending on their temperature. If you look closely at stars in the night sky, you can see some of these different colors (and they can be quite stunning through binoculars or a small telescope).

Absorption Lines

The hydrogen cloud absorbs some red light, leaving a valley in the spectrum on the left.

However, not all the light that stars give off reaches us. Some is absorbed by the gas in the star's atmosphere. Look at the animation at the right. The inner layers of the star give off all the colors of the rainbow, in different amounts. As the light passes through a hydrogen cloud in the outer layers of the star, some red light is absorbed by the hydrogen. Because the red light is absorbed, not all the light makes it to Earth, at the left side of the animation. The star's spectrum, shown at the left, has a valley in the red area. This is the cause of the valleys you saw in the spectra in the last exercise.

Hydrogen will only absorb light if the star is at the right temperature. If the star is too hot or too cool, the hydrogen will not absorb this light.

Sometimes, hydrogen clouds in stars give off as well as absorb light. When a hydrogen cloud gives off light, it gives off light exactly the same color as the cloud absorbs. If the cloud gives off more light than it absorbs, the spectrum at the left will have a peak instead of a valley. These peaks are called "emission lines," because the cloud emits light. Emission lines also depend on the temperature of a star; they only appear if a star is at the right temperature.

If you saw a cloud whose spectrum showed no hydrogen absorption or emission lines, how would you tell if it were hot or cool? For a cloud of pure hydrogen, you couldn't. But for real stars, which contain atoms of many elements besides hydrogen, you could look at the absorption and emission lines of other elements.

Absorption Lines in Real Stars

Most elements absorb or emit light best at a certain temperature; therefore, at that temperature, their absorption or emission lines are strongest. The lines you see in a star's spectrum act like thermometers. Some compounds, like titanium oxide, only appear in the spectra of very cool stars. Others, like helium, appear only in the spectra of very hot stars.

The sequence of spectral types, OBAFGKM, is actually a temperature sequence with O representing the hottest stars and M representing the coolest stars.

Here are some useful devices to remember the order of the spectral types:

The table below shows some of the characteristic absorption and emission lines of each star.

Spectral Type

Temperature (Kelvin)

Spectral Lines


28,000 - 50,000

Ionized helium


10,000 - 28,000

Helium, some hydrogen


7500 - 10,000

Strong hydrogen, some ionized metals


6000 - 7500

Hydrogen, ionized calcium (labeled H and K on spectra) and iron


5000 - 6000

Neutral and ionized metals, especially calcium; strong G band


3500 - 5000

Neutral metals, sodium


2500 - 3500

Strong titanium oxide, very strong sodium

You may not know where all of these elements have their absorption and emission lines. The chart below lists some of the more common ones and their approximate location in the electromagnetic spectrum.

Spectral Lines

Wavelengths (Angstroms)

Ha, Hb, Hg

6600, 4800, 4350

Ionized Calcium:
H and K Lines

3800 - 4000

Titanium Oxide

lots of lines from 4900 - 5200, 5400 - 5700, 6200 - 6300, 6700 - 6900

G Band




Helium (neutral)


Helium (ionized)


If you are interested in learning where to find all the lines the SDSS software uses, you can find a table of all the lines.

Question 4. How does your classification system compare to the OBAFGKM spectral type classification shown above? What are the similarities? What are the differences?

Now, take a look at the spectrum you saw earlier:

Click on the image to see it full size


Question 5. What lines are present in this spectrum? Do you see any spectral lines of ionized atoms?

Question 6. What is the spectral type of this star?

Got your answer? Click Next to see how you did!