We’ve already had our first snow and hard freeze here in Cheyenne, which means winter is definitely on its way. The summer constellations still dazzle in the west but will be sinking lower every day. The bright stars of the Summer Triangle; Deneb, Vega and Altair are easily picked out in the western sky. Facing east you’ll find Pegasus high in the sky – most easily picked out by its Great Square with the glint pointing you to the Andromeda Galaxy found nearby. Near the eastern horizon is the red star, Aldebaran, the eye of Taurus the Bull. Above Aldebaran is the Pleiades or Seven Sisters, always a favorite of many stargazers. It looks like a tiny little dipper but it is not the Little Dipper. Looking overhead this month you’ll find Cassiopeia, Cepheus and the Little Dipper. The Big Dipper is sitting upright on the northern horizon.
Some special things to watch for this month are a spectacular conjunction between the brilliant planets Venus and Jupiter and the peak of the Leonid meteor shower. The Venus and Jupiter conjunction will be obvious in the southwest, even in early twilight and become very obvious once the sky gets dark. The Leonid meteor shower peaks on Nov. 17 with about 15 meteors an hour. For those of you who have a solar telescope, Mercury will transit the Sun on Monday morning.
Have you ever wondered how our Sun compares to all the other suns we see sprinkled as stars across the night time sky? Stars, like people, come in all sizes with varying degrees of riches and brightness.
Stars have life cycles; they are born, live out their allotted life span, and then die. There are giant stars who, if put where our sun is would fill the area out past the orbit of Mars. There are stars the size of Earth, and stars the size of a city. There are red stars, blue stars, yellow stars and white stars. There are stars so thin that they are almost a vacuum. There are stars whose cores are made of diamond. Every element heavier than helium and hydrogen was created in a star somewhere. Stars have a predetermined lifetime and nuclear fusion is the energy source that allows them to shine. The eventual fate of a star is determined by its mass. Stars don’t change fast enough for us to see in our lifetime, so we try to figure out what stage of stellar evolution applies to each star we see.
Stars are formed by the collapse of great clouds of gas and dust called bok globules. The collapse of these interstellar nurseries begins through some force of gravity or compression as clouds collide. The cloud then collapses under its own weight, breaking up into smaller components. These smaller units become protostars and continue to collapse until their internal temperature reaches above 10 million degrees. The pressure of the gas in the protostar’s core then increases rapidly, halting the contraction causing nuclear fusion to start and a star is born. The nuclear fusion process is where a star, made primarily of hydrogen, begins fusing its atoms into the heavier atoms of helium.
Over 100 years ago astronomers classified stars according to how bright they appeared and their color. Today we know the color of a star is a direct indicator of the temperature of that star. The surface temperature of stars range from 2000 degrees Kelvin to over 50,000 degrees Kelvin. The hottest stars are blue and the coolest stars are red. Astronomers assign stars a spectral class as an easy way to tell the colors and temperatures of stars. The spectral classes, in order from hottest to coolest stars, are O, B, A, F, G, K, M. An informative and orderly diagram came into use in the early 1900s, which is known as the HR diagram. It shows the luminosity or brightness of a star charted along one axis and the temperature of a star along the other axis. Every star is represented as a point on the HR diagram and it reveals a distinctive pattern. The main sequence is a band along which you’ll find 90% of all stars. The main sequence is the stage where a star will spend the majority of its life. This “middle age” stage occurs while the star is burning hydrogen. How long a star stays on the main sequence depends on its mass. The more massive stars will live and die faster than less massive stars.
Luminosity class tells us the sizes of stars. The classes are I (bright supergiant), II (supergiant), III (giant), IV (subgiant) and V (main sequence). Supergiant stars are hundreds of times larger than our Sun. One of the largest stars known is Betelgeuse, a star that is believed to be about 1,000 times larger than our Sun. Giants are tens of times larger than our Sun. Red dwarfs are around the size of Jupiter and contain from 1/10 to 4/10 the mass of the Sun. Supergiant stars are rare; they make up about only 1% of the total number of stars. Red dwarfs are probably the most common type of stars, but would be difficult to see at a great distance. Our Sun is a yellow main sequence star.
Once a star exhausts its hydrogen supply, it leaves behind a helium core. The star then evolves off the main sequence to become a red giant as the hydrogen left in the outer portions of the star starts burning in a process called hydrogen shell burning. The star begins to increase in size and luminosity until it becomes a vast red giant producing 1,000 times the light of the sun. The star expands well over 100 times, causing the star to cool off. Once the hydrogen shell burning is finished, the helium core will start nuclear fusion again fusing heavier elements such as carbon, oxygen, neon and magnesium, then finally iron depending on how much mass the star contains. When the star begins fusing heavier elements in its core, the core will contract. When each element is fused, the shell burning will follow, causing the star to expand again as a red giant. How many red giant loops a star has is again determined by its mass.
A star that starts out with less than .1 solar masses never really gets hot enough to start hydrogen burning. It becomes a brown dwarf. A brown dwarf only gives off thermal radiation and it will slowly lose its heat until it becomes a cold cinder in space.
A star that starts with .1 to .4 solar masses will become a red dwarf. It will stay on the main sequence for over 200 billion years and eventually cool off to become a black dwarf, a cold cinder in space.
A star that starts with about 1 solar masses will stay on the main sequence about 10 billion years. It will become a red giant star and shed excess mass through a planetary nebula. This is where the outer atmosphere from the red giant phase expands away from the star. The collapsed core of the star then becomes a white dwarf. The white dwarf will eventually cool off until it becomes a black dwarf. Our Sun will end its life this way, but don’t worry, it has about another 5.4 billion years to go before it starts to evolve into a red giant.
A star that starts out with 5 to 10 solar masses will remain on the main sequence for about 500 million years. It will go through red giant loops, fusing heavier and heavier elements and also shed mass through planetary nebula, stellar winds, supernova or nova. If less than 1.4 solar masses is left in the stellar remnant, it will become a white dwarf. If between 1.4 solar masses to 3 solar masses is left, it will become a neutron star. If more than three solar masses is left, it will become a black hole.
A star starting with more than 20 solar masses will stay on the main sequence for only a few million years. It will fuse heavier elements all the way down to iron. It will shed mass through stellar winds and supernova explosions. If less than 1.4 solar masses remain, it will become a white dwarf. If between 1.4 to 3 solar masses remain, it will become a neutron star, and if more than 3 solar masses remain, it will become a black hole.
It is thought that stars as massive as 100 suns could have been born but they would most likely have exploded almost instantly.