Stars

FAS Astronomers Blog, Volume 30, Number 5.

Stars are huge balls of hydrogen plasma powered by nuclear fusion reactions at their core.

Stellar Distances

Except for the Sun, which is 93 million miles away, stars are a vast distance from us. Therefore, it isn’t always practical to measure these distances in miles, so astronomers use other units.

• An astronomical unit (au) is the average distance of the Earth from the Sun (93 million miles).
• A light year is the distance light travels in a year (5.88 trillion miles). This is based on the speed of light in a vacuum, which is 300,000 km/sec or 186,000 miles/sec.
• A parsec (PARalax SECond) is the distance at which a star’s parallax is 1 arcsecond when using the Earth’s orbit as a base (3.26 light years). A megaparsec is a million parsecs (3.26 x 10⁶ light years).

The nearest star to the Sun is Proxima Centauri at 4 ¼ light years (25 trillion miles) away. As a comparison, Neptune, the last official planet in the Solar System, is a mere 2.8 billion miles away from the Sun. Mars can be as close as 40 million miles from the Earth.

Most of the stars we see in the night sky are in the general vicinity of the Sun (up to one or two thousand light years away). For example, Sirius, the brightest star in the night sky, is 8.7 light years away, and Vega, another bright star, is 25 light years away. Deneb, a star in the summer triangle, is much farther at over one thousand light years.

If you get away from the city lights on a dark night, you might see a faint band of light stretching across the sky. This is our galaxy, the Milky Way, which contains a few hundred billion stars. Our solar system is located around ½ way out from the center of the Milky Way (~26,000 light years away). The portion of the Milky Way that we see is the neighboring arms, which are closer at several thousand light years away.

Stars are found in galaxies and there are countless galaxies in the visible universe. I’ve seen estimates of a 100 billion up to a trillion. If we assume there are 200 billion stars in a typical galaxy and 200 billion galaxies, then there are 4 x 10²² stars in the visible universe. Galaxies are much further from us than the stars we see at night. The Andromeda galaxy is one of the closest, yet it is 2 ½ million light years away. Other galaxies can be found millions or billions of light years away.

Names of Stars

The brighter stars have proper names (e.g., Sirius, Vega, Cappella, and Betelgeuse). Many of these names have Arabic roots (e.g., Betelgeuse, Aldebaran, Dubhe, Alintak, Alnilam, and Mintaka). This is from a time when astronomy was kept alive in the Middle East during the “Dark Ages” in Europe.

Stars are classified using the Bayer designation, where the brightest star in each constellation is given the first Greek letter (a), the next brightest the second Greek letter (b), and so on. Although, there are a few exceptions where the second brightest star is designated alpha (e.g., Castor and Betelgeuse). There is also the Flamsteed designation where stars within a constellation are assigned a number based on their right ascension. Just to make it more confusing, these names use the Latin genitive and not the name of the constellation (for example, Betelgeuse is Alpha Orionis and not Alpha Orion).

Stars are also named for the catalog they are found in. A few of the more well know are the Henry Draper catalogue (HD prefix), the Gliese catalog (GL prefix), the Gliese–Jahreiß catalog (GJ prefix), the Yale Catalog of Bright Stars (HR prefix), and the Smithsonian Astrophysical Observatory Star Catalog (SAO prefix).

Lifecycle of Stars

Our Sun is a middle-aged star that’s been around for about 4 ½ billion years. It is said to have a high(er) “metallicity”. That is, it contains a higher percentage of elements other than hydrogen and helium. Population I stars like the Sun are found in the disk of the Milky Way and in open star clusters such as the Pleiades. Jan Oort (1926) and Walter Baade (1944) recognized that there is a second group of stars with low(er) metallicity. These population II stars are found near the center of the galaxy and in the galactic halo including globular clusters. Astronomers have also theorized that a third group of stars, population III with zero metallicity, might have existed in the early universe. These stars would have been composed only of elements forged in the big bang (hydrogen and helium with a slight amount of lithium).

Stars form from large clouds of gas and dust that compress due to gravity. Over a long period of time, gravity squeezes the material into a ball creating a protostar. Eventually, the ball becomes dense enough and hot enough, so nuclear fusion begins, and a star is formed. At this point, fusion in the star’s core converts hydrogen into helium releasing a huge amount of energy. This energy can take over one hundred thousand years to work its way to the surface of the star where it is released as visible light.

For most of their lives, while burning hydrogen in their core, stars maintain a balance between two forces:

• The force of gravity trying to collapse the star.
• The nuclear reactions in star’s core pushing outward.

Stars eventually use up their supply of hydrogen fuel. When the hydrogen in their core runs out, stars expand into a red giant (or supergiant), burn heavier elements in their core, and hydrogen in their shell. The temperature in the core increases, while the outer layers expand and, because of a larger surface area, decrease in temperature. What happens depends on the star’s size.

After burning hydrogen in their core, low mass stars will start to burn helium and the star’s temperature and pressure increases causing its outer layers to expand into a red giant. When the supply of helium is exhausted, the final collapse begins. The core becomes a white dwarf, while the outer shell is expelled creating what we call a planetary nebula. The white dwarf stage is a balance between the inward force of gravity and the outward force created by “degenerate” electrons that can no longer be pushed together. In some cases, a white dwarf star will pull matter from a companion star. It will eventually explode as a type IA supernova when it reaches a size of around 1.4 solar masses (The Chandrasekhar Limit).

More massive stars will burn helium, followed by heavier elements (carbon, oxygen, …). The core will heat up more and more and the pressure will become greater and greater. This causes the star’s outer layers to expand into a supergiant. Once the core becomes mostly iron, the process stops. The core then collapses very quickly, and the temperature reaches billions of degrees. A shock wave results, ejecting stellar material into space, which we see as a type II supernova. During the recoil, material is heated, and heavier elements are formed. A remnant core is left over. For stars with lighter cores, the nuclear force maintains a balance with gravity and a neutron star is formed. For stars with heavier cores, gravity overcomes the nuclear force, and a black hole is formed.

Heavier elements are forged in supernovae (both types Ia and II), but that isn’t the entire story. Many heavy elements are also formed during the expansion of stars into red giants and in the merger of neutron stars (see The Origin of the Elements). The elements are created through two processes called rapid (r-process) and slow (s-process) neutron capture rather than nuclear fusion found in the core of stars. Neutrons are captured and then converted into protons via Beta decay. This increases the atomic number and a new element is formed.

Measuring Stellar Brightness

A star might be bright in the night sky because it is large and luminous (e.g., Deneb) or because it is simply close to us (e.g., Sirius). It is difficult to tell just by looking at the star and measuring stellar distances isn’t all that straight forward. Astronomers use various “yard sticks” such as parallax, Cepheid variables, and type 1a supernovae to gauge cosmic distances.

A star’s luminosity (L) is a measure of the light emitted by the star. Technically, luminosity is the energy per second produced by the star, and it is proportional to the star’s radius² and the temperature⁴. However, it is most often stated as a ratio to the luminosity of the Sun (L_o).

The magnitude of a star is an alternative measurement and uses a scale ranging from negative numbers (brightest) to positive numbers (dimmest). The difference in one magnitude is 2.512 times the ratio in brightness.

• Apparent Magnitude (m) is the brightness of the star as seen from the Earth.
• Absolute Magnitude (M) is the brightness of the star from the fixed distance of 10 parsecs (32.6 light years).

For more on this, see Cosmic Distances, Stellar Brightness, and The Hubble Constant.

Classification of Stars

Stars come in all sizes and colors. For many years, astronomers thought that VY Canis Majoris was the largest star at around 1,500 times the diameter of the Sun. Recently this title has been taken by UY Scuti, which is believed to have a diameter about 1,700 times that of the Sun. The most massive star is R136a1 at 265 times the mass of the Sun. You might ask, “what is the smallest star?” We just don’t know; it is probably too small to be noticed.

Stars are classified using the Morgan-Keenan Classification, which was originally proposed by Cecilia Payne in the early 1900s (see The Harvard Computers). Stars are assigned one of seven letters: O, B, A, F, G, K, M (Oh Be A Fine Guy/Girl Kiss Me) based on their color and temperature. O stars are blue, hot, and large. M stars are red, cool, and small. The classification used today is more detailed and includes a numerical designation for each letter to further define the spectral type. There is also a third character (Ia, Iab, Ib, II, III, IV, V) used to designate the star’s luminosity (ranging from supergiants to giants down to main-sequence stars). As an example, our Sun is a type G2V star.

Stars are found to have a relationship between their luminosity, temperature, and color, which is shown in the Hertzsprung-Russell (H-R) diagram. Surface temperature (high to low)/Stellar Classification is displayed on the horizontal axis and luminosity (low to high) is on the vertical axis. Stars remain on the main sequence of the H-R diagram for most of their life while they burn hydrogen in their core. Larger hotter (bluer) stars are to the upper left and cooler smaller (reddish) stars are to the lower right. Yellow/White stars like the Sun are in the middle. Older/larger red giant stars are found in the mid to upper right and older/smaller white dwarfs are in the lower left.

Although they are not visible in the night sky, most stars are the smaller red dwarfs (Class M). This is probably because the larger/hotter stars (Class O and B) burn through their hydrogen fuel very quickly and only live a short time (several million years), while the small/cooler stars can live for trillions of years. As a comparison, we expect the Sun to live around ten billion years.

The Brightest Stars

Below is a list of the brightest stars as seen from the Earth with some of their attributes (as found in Wikipedia). Other sources are listed below. However, their published stellar characteristics don’t always match Wikipedia or each other.

For more about stars, see Betelgeuse is Dimming, The Harvard Computers, The Night Sky, Observing the Stars, Cosmic Distances, Stellar Brightness, and The Hubble Constant, and The Sun.