It's a dark, clear, moonless night. You look up into the sky. You see thousands of stars arranged in patterns or constellations. The light from these stars has traveled great distances to reach Earth. But what are stars? How far away are they? Are they all the same? Are there other planets around them?

Photo courtesy of NASA
The Milky Way Galaxy

In this edition of How Stuff Works, we will look at the fascinating world of stars. We will examine the nature of stars, types of stars, how stars form and how stars die. If you have read How the Sun Works, you already know a lot about the nature of Earth's nearest star. As you read the following pages, you'll find out even more about what you can see in the night sky.

Stars and Their Properties
  • absolute magnitude - apparent magnitude of the star if it was located 10 parsecs from Earth
  • apparent magnitude - a star's brightness as observed from Earth
  • luminosity - total amount of energy emitted from a star per second
  • parsec - distance measurement (3.3 light-years, 19.8 trillion miles, 33 trillion kilometers)
  • light year - distance measurement (6 trillion miles, 10 trillion kilometers)
  • spectrum - light of various wavelengths emitted by a star
  • solar mass - mass of the sun; 1.99 x 1030 kg (330,000 Earth masses)
  • solar radius - radius of the sun; 418,000 miles (696,000 km)
Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are relatively close (the closest 30 stars are within 40 parsecs) and others are far, far away. Astronomers can measure the distance by using a method called parallax, in which the change in a star's position in the sky is measured at different times during the year. Some stars are alone in the sky, others have companions (binary stars) and some are part of large clusters containing thousands to millions of stars. Not all stars are the same. Stars come in all sizes, brightnesses, temperatures and colors. Let's take a closer look at the features of stars.

Stars have many features that can be measured by studying the light that they emit:

  • temperature
  • spectrum or wavelengths of light emitted
  • brightness
  • luminosity
  • size (radius)
  • mass
  • movement (toward or away from us, rate of spin)

Temperature and Spectrum
Some stars are extremely hot, while others are cool. You can tell by the color of light that the stars give off. If you look at the coals in a charcoal grill, you know that the red glowing coals are cooler than the white hot ones. The same is true for stars. A blue or white star is hotter than a yellow star, which is hotter than a red star. So, if you look at the strongest color or wavelength of light emitted by the star, then you can calculate its temperature (temperature in degrees Kelvin = 3 x 106/ wavelength in nanometers). A star's spectrum can also tell you the chemical elements that are in that star because different elements (for example, hydrogen, helium, carbon, calcium) absorb light at different wavelengths.

Brightness, Luminosity and Radius
When you look at the night sky, you can see that some stars are brighter than others as shown in this image of Orion.

Photo courtesy of NASA
The constellation Orion as seen from the space shuttle Endeavour (STS-54)

Two factors determine the brightness of a star:

  • luminosity - how much energy it puts out in a given time
  • distance - how far it is from us
A searchlight puts out more light than a penlight. That is, the searchlight is more luminous. If that searchlight is 5 miles away from you, however, it will not be as bright because light intensity decreases with distance squared. A searchlight 5 miles from you may look as bright as a penlight 6 inches away from you.The same is true for stars.

Stefan-Boltzmann Law

This is the relationship between luminosity (L), radius (R) and temperature (T):

L = (7.125 x 10-7) R2 T4

Units: L - watts, R - meters, T - degrees Kelvin

Astronomers (professional or amateur) can measure a star's brightness (the amount of light it puts out) by using a photometer or charge-coupled device (CCD) on the end of a telescope. If they know the star's brightness and the distance to the star, they can calculate the star's luminosity [luminosity = brightness x 12.57 x (distance)2].

Luminosity is also related to a star's size. The larger a star is, the more energy it puts out and the more luminous it is. You can see this on the charcoal grill, too. Three glowing red charcoal briquettes put out more energy than one glowing red charcoal briquette at the same temperature. Likewise, if two stars are the same temperature but different sizes, then the large star will be more luminous than the small one. See the sidebar for a formula to that shows how a star's luminosity is related to its size (radius) and its temperature.

Mass and Movement
In 1924, the astronomer A. S. Eddington showed that the luminosity and mass of a star were related. The larger a star (i.e., more massive) is, the more luminous it is (luminosity = mass3).

Stars around us are moving with respect to our solar system. Some are moving away from us and some are moving toward us. The movement of stars affects the wavelengths of light that we receive from them, much like the high pitched sound from a fire truck siren gets lower as the truck moves past you. This phenomenon is called the Doppler effect. By measuring the star's spectrum and comparing it to the spectrum of a standard lamp, then the amount of the Doppler shift can be measured. The amount of the Doppler shift tells us how fast the star is moving relative to us. In addition, the direction of the Doppler shift can tell us the direction of the star's movement. If the spectrum of a star is shifted to the blue end, then the star is moving toward us; if the spectrum is shifted to the red end, then the star is moving away from us. Likewise if a star is spinning on its axis, the Doppler shift of its spectrum can be used to measure its rate of rotation.

So you can see that we can tell quite a bit about a star from the light that it emits. Furthermore, amateur astronomers today have devices like large telescopes, CCDs and spectroscopes commercially available to them at relatively low cost. Therefore, amateurs can do the same types of measurements and stellar research that used to be done by professionals alone.

Classifying Stars: Putting the Properties Together
In the early 1900s, two astronomers, Annie Jump Cannon and Cecilia Payne, classified the spectra of stars according to their temperatures. Cannon actually did the classification and Payne later explained that a star's spectral class was indeed determined by the temperature.

Spectral Classes of Stars
Spectral Class
Ave.Temp. (K)
Familiar Examples
OBlue-violet30,000Mintaka (delta Orionis)
BBlue-white20,000Rigel, Spica
AWhite10,000Vega, Sirius
FYellow-white8,000Canopus, procyon
GYellow6,000Sun, Capella
KOrange4,000Arcturus, Aldebaran
MRed-orange3,000Antares, Betelgeuse

In 1912, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently graphed the luminosity vs. temperatures for thousands of stars and found a surprising relationship as shown below. This diagram called a Hertsprung-Russell or H-R diagram revealed that most of the stars lie along a smooth diagonal curve called the main sequence with hot, luminous stars in the upper left and cool, dim stars in the lower right. Off of the main sequence, there are cool, bright stars in the upper right and hot, dim stars in the lower left.

Photo courtesy of NASA
The Hertzsprung-Russell Diagram. The Sun, 12 brightest stars of the Northern Hemisphere and white dwarf companion stars to Sirius and Procyon are shown.

If we apply the relationship between luminosity and radius to the H-R diagram, we find that the radius of the stars increases as you proceed bottom left diagonally to top right:

  • Sirius B = 0.01 solar radius
  • Sun = 1 solar radius
  • Spica = 10 solar radii
  • Rigel = 100 solar radii
  • Betelgeuse = 1000 solar radii
If you apply the relationship between mass and luminosity to the H-R diagram, you find that stars along the main sequence vary from the highest (approximately 30 solar masses) at the top left to the lowest (approximately 0.1 solar mass) at the bottom right. As you can see from the H-R diagram, our Sun is an average star.

The H-R diagram summarizes the types of stars in the universe:

Classes of Stars by Luminosity
Familiar Examples
IaBright SupergiantsRigel, Betelgeuse
IbSupergiantsPolaris (the North star), Antares
IIBright GiantsMintaka (delta Orionis)
IIIGiantsArcturus, Capella
IVSub-giantsAltair, Achenrar (a Southern Hemisphere star)
VMain sequenceSun, Sirius
not classifiedWhite dwarfsSirius B, Procyon B

White dwarfs stars are not classified because their stellar spectra are different from most other stars. The H-R diagram is also useful for understanding the evolution of stars from birth to death.

The Life of a Star
As we mentioned before, stars are large balls of gases. New stars form from large, cold (10 degrees Kelvin) clouds of dust and gas (mostly hydrogen) that lie between existing stars in a galaxy.

  1. Usually, some type of gravity disturbance happens to the cloud such as the passage of a nearby star or the shock wave from an exploding supernova.
  2. The disturbance causes clumps to form inside the cloud.
  3. The clumps collapse inward drawing gas inward by gravity.
  4. The collapsing clump compresses and heats up.
  5. The collapsing clump begins to rotate and flatten out into a disc.
  6. The disc continues to rotate faster, draw more gas and dust inward, and heat up.
  7. After about a million years or so, a small, hot (1500 degrees Kelvin), dense core forms in the disc's center called a protostar.
  8. As gas and dust continue to fall inward in the disc, they give up energy to the protostar, which heats up more
  9. When the temperature of the protostar reaches about 7 million degrees Kelvin, hydrogen begins to fuse to make helium and release energy.
  10. Material continues to fall into the young star for millions of years because the collapse due to gravity is greater than the outward pressure exerted by nuclear fusion. Therefore, the protostar's internal temperature increases.
  11. If sufficient mass (0.1 solar mass or greater) collapses into the protostar and the temperature gets hot enough for sustained fusion, then the protostar has a massive release of gas in the form of a jet called a bipolar flow. If the mass is not sufficient, the star will not form, but instead become a brown dwarf.
  12. The bipolar flow clears away gas and dust from the young star. Some of this gas and dust may later collect to form planets.
The young star is now stable in that the outward pressure from hydrogen fusion balances the inward pull of gravity. The star enters the main sequence; where it lies on the main sequence depends upon its mass.

Photo courtesy of NASA
Gas pillars in a star-forming region - M16 (Eagle Nebula)

Now that the star is stable, it has the same parts as our Sun:

  • core - where the nuclear fusion reactions occur
  • radiative zone - where photons carry energy away from the core
  • convective zone - where convection currents carry energy toward the surface
However, the interior may vary with respect to the location of the layers. Stars like the Sun and those less massive than the Sun have the layers in the order described above. Stars that are several times more massive than the Sun have convective layers deep in their cores and radiative outer layers. In contrast, stars that are intermediate between the Sun and the most massive stars may only have a radiative layer.

Life on the Main Sequence
Stars on the main sequence burn by fusing hydrogen into helium. Large stars tend to have higher core temperatures than smaller stars. Therefore, large stars burn the hydrogen fuel in the core quickly, whereas, small stars burn it more slowly. The length of time that they spend on the main sequence depends upon how quickly the hydrogen gets used up. Therefore, massive stars have shorter lifetimes (the Sun will burn for approximately 10 billion years). What happens once the hydrogen in the core is gone depends upon the mass of the star.

The Death of a Star
Several billion years after its life starts, a star will die. How the star dies, however, depends on what type of star it is.

Stars Like the Sun
When the core runs out of hydrogen fuel, it will contract under the weight of gravity. However, some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up. This heats the upper layers, causing them to expand. As the outer layers expand, the radius of the star will increase and it will become a red giant. The radius of the red giant sun will be just beyond the Earth's orbit. At some point after this, the core will become hot enough to cause the helium to fuse into carbon. When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject material that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years.

Photo courtesy of NASA/Space Telescope Science Institute
Hubble Space Telescope photograph of the Rotten Egg planetary nebula

Stars More Massive Than the Sun
When the core runs out of hydrogen, these stars fuse helium into carbon just like the Sun. However, after the helium is gone, their mass is enough to fuse carbon into heavier elements such as oxygen, neon, silicon, magnesium, sulfur and iron. Once the core has turned to iron, it can burn no longer. The star collapses by its own gravity and the iron core heats up. The core becomes so tightly packed that protons and electrons merge to form neutrons. In less than a second, the iron core, which is about the size of the Earth, shrinks to a neutron core with a radius of about 6 miles (10 kilometers). The outer layers of the star fall inward on the neutron core, thereby crushing it further. The core heats to billions of degrees and explodes (supernova), thereby releasing large amounts of energy and material into space. The shock wave from the supernova can initiate star formation in other interstellar clouds. The remains of the core can form a neutron star or a black hole depending upon the mass of the original star.

Photo courtesy of NASA/Space Telescope Science Institute
Hubble Space Telescope photograph of the rings around Supernova 1987A

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