Astronomers have a neat trick up their sleeves when it comes to approximating the size of a star. This lesson will tell you how they do it and how the sizes are grouped.
How Stars Can Be Classified
The spectra of stars are like a product barcode. I used to work at a place where I had to scan barcodes. When I did, it would tell me what the product was, where it had come from, and where it needed to go.Like a barcode told me about the product I was scanning, a star’s spectrum can tell an astronomer different things as well.
It can tell them about the chemical composition of the star, what its temperature is, as well as what kind of star it is. The former two are covered in other lessons, as we talk about how stellar spectra help astronomers classify the different kinds of stars and what luminosity classes have to do with it.
Atoms and Spectral Lines
When I scanned different products, I noticed that not all barcode lines were created equal. Some lines were really thin and others were really broad. All of that signified something to the computer analyzing those lines.
When an astronomer analyzes a star’s spectrum and finds broad spectral lines, it tells them something important. In a dense gas, atoms collide more frequently. This distorts their energy levels in such a way as to produce broader spectral lines. Main-sequence stars, adult stars lying on the main sequence of the HR diagram, have broad spectral lines because their atmospheres are dense.Giant stars are less dense. Their atoms collide less frequently and the spectral lines are narrower.
Supergiants are even less dense and their spectral lines are very narrow.You get the idea. Lower density equals narrower spectral lines.
Unfortunately, I never figured out the meaning of what broad or narrow lines on the barcodes I scanned meant and was subsequently fired as a result. I’m only kidding.
I quit.You can actually eyeball a star’s spectrum and be able to tell how big it is. The size of a star as derived from a star’s spectrum is known as a luminosity class.
The word ‘luminosity’ is used because the more luminous a star is, the larger its size has to be. Luminosity is the total energy a star radiates in one second. Like a big fire pushes you away with all the hot energy it radiates, a bigger star will have a larger luminosity.The luminosity classes can be simplified into the following:
- Ia for luminous supergiants, such as Rigel
- Ib for supergiants, such as Polaris (the North Star)
- II for a luminous giant, like Adhara
- III for a giant, as per Aldebaran
- IV for a subgiant, for which Altair is an example
- V for a main-sequence star, which is the best class in my opinion because our sun is such a star!
The larger the Roman numeral, the denser the star, and the broader the spectral lines.
Let’s summarize what we learned here.
Barcode lines cannot be analyzed by anything other than a machine. That was the most important thing.Other than that, some less important stuff went something like this:Dense gas produces broader spectral lines.
So main-sequence stars will have broader lines than supergiants. Main-sequence stars are adult stars lying on the main sequence of the HR diagram. You can thus eyeball a star’s spectrum and approximate its size.The size of a star as derived from a star’s spectrum is known as a luminosity class. The word ‘luminosity’ is used because the more luminous a star is, the larger its size.
Luminosity is the total energy a star radiates in one second.The luminosity classes are simplified for you to this:
- Ia for luminous supergiants
- Ib for supergiants
- II for a luminous giant
- III for a giant
- IV for a subgiant
- V for a main-sequence star
Use this lesson to strengthen your capacity to:
- State the definitions of main-sequence stars and luminosity
- Point out the relationship between gas density and spectral lines
- Explain what luminosity class is and identify the six classes and an example of each