During the year, we will examine the many different types of stars in the Universe.
This week, let’s start with the brightest star in the sky: the sun.
We all know that our sun is a star. We learn this fact as children and it doesn’t seem to be terribly profound. Yet, this fact would have astonished sky-watchers of old. Until the time of Newton and Galileo, most people divided the universe into two categories, the terrestrial and ethereal. The terrestrial world was occupied by everything comfortable and familiar, like trees, cathedrals, huts, and falling apples. Stars, planets, sun and moon occupied the ethereal plane. Although the sun was securely placed within its own crystalline sphere, it was perceived to be a special entity, wholly separate from the enigmatic stars beyond.
When humanity realized that the Earth was revolving around a sun and that the sun was just one of many stars, astronomers were determined to unveil its secrets.
We often say that the sun is an average star: not excessive or diminutive and not too hot or too cool. It’s a Goldilocks star.
Yet, we must be careful when we start defining the sun as average, for we must first determine the parameters we’ll use to define our standards. A parameter is what we decide it should be. For instance, if we were trying to gauge the size of a state, we’d have to determine if we were going to rate them according to land area or population. Alaska would be at the top of the list in terms of land area, but would be assigned a lower ranking if measured in terms of population.
Similarly, the sun’s ranking in the universe of stars differs depending on the parameters.
Now, if you take all the stars in our galaxy and line them up according to size, the sun would rank near the top. Ninety percent of the Milky Way’s stars are red dwarfs, stars that are just hot enough to ignite and sustain thermonuclear reactions. At the opposite end of the spectrum we find the super and hyper-giants. Relatively few of these highly massive stars are found within any galaxy. Though there are millions of stars that are larger than the sun, there are billions of smaller stars.
Let’s measure the sun’s range using stellar surface temperature. The coolest red stars have effective temperatures of 2000 to 2500 degrees. The massive stars Betelgeuse and Antares are cool, red stars. The hottest blue-white stars have temperatures that equal or exceed 50,000 degrees! Our yellow sun’s surface temperature is 6,000 degrees, making it at the cooler end of the temperature spectrum.
What if we simply considered the stellar size spectrum, and not the individual sizes of all the stars? Here we define star as a gaseous sphere that is able to sustain core fusion reactions. Models show us that such fusion stars must be at least 8 percent as massive as the sun. These stars are the smallest. The largest star, the hyper-giants, can be 100 times as massive as Sol. Yet, these 100-solar-mass stars are quite rare. Only a few exist in the Milky Way.
By definition, the sun is one solar mass. The range of possible masses begins at 0.08 and extends to 100.
Finally, let’s look at stellar lifetimes. Counter-intuitively, the least massive stars have the longest life spans. Hyper-giants are generally unstable and will live for only a few million years, while the more placid red-dwarfs can exist for more than a trillion years…far longer than the current age of the universe. Our sun has been around for five billion years. Astronomers predict that it will remain in its current state for another five billion. Taking into account the red giant and white dwarf stages which are destined to eventually occur, we believe that the sun’s life-span is 13 billion years. It could pass through its whole life and then die 100 times over before even one red dwarf would show signs of age. Conversely, the sun’s life span equals that of 1000 hyper-giants.