Stars

Characteristics of Stars A star is a massive ball of plasma that emits light and electromagnetic energy throughout the universe. While there is only one star in our solar system, there are billions upon billions of stars throughout our galaxy and exponentially more in the billions of galaxies in the universe. Stars are very far away and extremely hot which makes it very difficult to obtain data on them. A star can be defined by five basic characteristics: surface temperature, composition, brightness, size, and distance from Earth.

Surface Temperature (Color) The color of a star indicates its temperature. Astronomers measure a star's temperature on the Kelvin scale. Zero degrees on the Kelvin scale is theoretically absolute and is equal to degrees Celsius. The coolest, reddest stars are approximately 2,500 K, while the hottest, blue stars can reach 50,000 K. Our sun is about 5,500 K and classified a yellow star.

Stars are classified according to their temperatures on a spectral classification chart. Our sun is classified as a type G star.

Composition Stars burn because of nuclear fusion. Hydrogen atoms fuse together to become helium atoms while giving off a lot of energy. Stars are made of primarily hydrogen and helium, but other elements exist as the star starts to age. The more hydrogen, the younger the star.

Brightness Brightness is the amount of light that a star gives off. Apparent magnitude of a star is its brightness as seen from Earth, factoring in size and distance. Absolute magnitude is its true or actual brightness, irrespective of its distance from earth. The Ptolemy Magnitude System assigns a magnitude number for a star’s brightness. The smaller the number, the brighter the star. 6 th magnitude stars are the faintest stars that can be seen from Earth with the naked eye (in an extremely dark area). Apparent and absolute magnitudes can be very different from Earth. The sun has an absolute magnitude of 4.8, but an apparent magnitude of because it is so close to the earth.

The smallest stars are the neutron stars which are very small, dense stars with a diameter of about 5-12 miles (5-16 km). These stars are the remnants of dead stars. Star Size Our sun is considered a dwarf star. Dwarfs can be red, yellow, or brown and can be about half the size of our sun to about 20 times larger. The largest stars are giant and supergiant stars. Giant stars can be 100 times larger than our sun while supergiants can be 1000 times larger than our sun. Supergiant stars are normally dying stars.

Distance from Earth It is difficult to measure the distance that a star is from the earth. Star distance is measure using parallax which is a star’s apparent shift in position over a period of time. Using the shifts made by stars and mathematical calculations, a star’s distance can be determined. Star distance is measured in light-years, which is the distance light travels in one year (about 6 trillion miles or 10 trillion km). You can demonstrate this shift, or parallax, easily. Hold up your finger at arm's length and alternately open and close each eye. Your finger will appear to move relative to objects in the background. For the stars, the parallactic shift is extremely tiny but it can be measured.

The closest star to us (besides Sun, of course) is the faint, red dwarf star called Proxima Centauri. This star is only 4.2 light-years away. This is actually a triple star system.

Life Cycle of a Star Stars are “born” out of clouds of dust and gases called a nebula. This area of dust and gas begins to spin and the gravitational pull of the gas allows nuclear fusion to occur and a protostar is “born”. Most of a star’s “life” is spent as a main-sequence star. A main-sequence star is a middle age star that has a brightness proportional to its temperature. With main-sequence stars, very hot stars tend to be very bright and very cool stars tend to be very dim. Our sun is a main-sequence star which has an average temperature and an average brightness.

This diagram that shows the relationship of a star’s magnitude to its temperature is called the Hertzsprung-Russell (or H-R) diagram. The H-R diagram not only shows the relationship in “healthy” main-sequence stars but also shows those stars that are in the process of dying: white dwarfs and red giants. Main-sequence stars can be very small (dwarf) stars to very large (giant) stars. Our sun has been burning for about 5 billion years and will continue to burn for approximately another 5 billion years..

The way that a star “dies” depends on how massive the star initially is. There are three basic masses of stars: dwarf stars (about the size of our sun), massive stars (100 times larger than our sun) and supermassive stars (1000 times larger than our sun).

Death of a Low Mass (Dwarf) Star After burning about 10 billion years, a dwarf star will finish its core burning and begin expanding. A dwarf star becomes hotter at its core, but cooler on its surface. Despite a cooler surface, due to its much larger surface area, it will be much brighter and will be classified as a red giant. Now classified a red giant, it migrates off the main- sequence portion of the H-R diagram to the upper right-hand corner of the very bright, but cooler stars. Our sun will become a red giant in about 5 billion years. At that time, the sun’s planets will undergo a change: the inner planets will be engulfed and vaporized by the new red giant and the outer planets will be heated which will lead to their atmospheres being burned off leaving the small rocky inner surface.

The helium core of the dwarf star continues to burning and converting the helium core into larger elements such as carbon and oxygen through nuclear fusion. Eventually a carbon core forms at the center of the star. Over time, the core of the star collapses and 25-60% of the dwarf’s outer gases are ejected forming a planetary nebula. A small, dense white dwarf is left at the center of the planetary nebula. This white dwarf will eventually use up all of its remaining fuel leaving the carbon core behind (black dwarf).

So our sun will eventually become a white dwarf that burns out becoming a black dwarf.

Death of a Massive and Supermassive Star The death of massive and supermassive stars proceed much like that of a dwarf star. Larger stars use up their energy more quickly than dwarf stars thus do not live as long. A massive or supermassive star will finish its core burning and begin expanding. The massive or supermassive star becomes hotter at its core, but cooler on its surface. Despite a cooler surface, due to its much larger surface area, it will be much brighter and will be classified as a red supergiant. Now classified a red supergiant, it migrates off the main-sequence portion of the H-R diagram to the upper right-hand corner of the very bright, but cooler stars.

The helium core of the massive or supermassive star continues to burning and converting the helium core into larger elements such as carbon and oxygen through nuclear fusion. But because of its size will continue converting those elements into iron atoms. The iron in the core of the star makes the core of the star very unstable. Eventually the massive or supermassive star will explode or supernova. If the star is a massive star, after the supernova, the remains of the star will collapse down to an area only about miles in diameter and be classified as a neutron star. Neutron stars are extremely small, hot, and dense.

If the star is a supermassive star, after the supernova, the gravity that remains is so great that the star collapses upon itself creating a black hole.

A black hole is formed when a star of sufficient mass undergoes gravitational collapse, with most or all of its mass compressed into a sufficiently small area of space, causing infinite space-time curvature at that point (a "singularity"). Such a massive space-time curvature allows nothing, not even light, to escape from the "event horizon," or border.

Recap of the Life Cycle of a Star