Stars of Different Colors

Marianne Dyson, September 2018

Visible in the evening starting this month, the two brightest stars of Orion are showing off their colors. But red Betelgeuse (Orion’s left shoulder, pronounced “beetle-juice”) and blue Rigel (Orion’s right foot, pronounced “rye-gel”) are destined to produce truly spectacular performances in the future.

Illustration of the constellation Orion from: AstroBob.areavoices.com

Big and Bright

People used to think that all stars are about the same size. Therefore, stars that appeared brighter must be closer like flashlights near versus farther away.

Then in 1905, astronomer Ejnar Hertzsprung used parallax (see July blog) to measure the distance to both bright and faint stars. Surprise! Some bright stars were the same distance as dim ones, and some much farther away. The reason? Some stars are brighter because they are physically bigger, like a floodlight versus a flashlight. This was proven correct in 1920 when astronomers measured the angular diameter of Betelgeuse using the (then) new 100-inch telescope on Mount Wilson. Betelgeuse is so large that if it replaced the sun, it would stretch out past the orbit of Jupiter. [Ref: European Southern Observatory.]

Betelgeuse is Cool

Betelgeuse is very obviously a different color than most stars. Human eyes see it as orangish red whereas Rigel looks blue. These colors aren’t just pretty, they reveal the temperature of the star.

Human eyes are good at judging heat output by color. Anyone who has ever roasted a marshmallow quickly discovers that a blue flame will burn it to a crisp whereas a warm yellow fire or a set of red embers will slowly brown it. Thankfully, we don’t have to hold marshmallows up to various stars to prove some are hotter than others. Scientists have quantified the colors by wavelengths so all we have to do is look at their spectra to tell precisely how hot stars are.

Human eyes only see a portion of a star’s total spectrum, aptly named the visible spectrum. The “coolest” end of the visible spectrum is red. The “hottest” is purple also called violet. (Physics students memorize the order: Red, Orange Yellow, Green, Blue, Indigo, Violet, as the name ROY G BIV.)

So just by looking at stars we can tell which ones are the coolest! Betelgeuse is a cool red. In the middle, temperature-wise, is our yellow sun. Blue Rigel is the hottest.  (See Bad Astronomy for why we don’t see green stars.)

Stellar spectra (seen via prisms or spectrometers) allow astronomers to measure the temperatures of stars. The surface of Betelgeuse is about 5800 degrees F, about half as hot as the Sun at 10,000 degrees. Rigel would vaporize our marshmallows long before we got close to its 36,000-degree surface. [Ref: Griffith Observatory.]

Why is Betelgeuse so cool?

Red in the End 

The temperature of a star depends mostly on its mass and age. Stars form by gravity pulling gas into a ball until it is hot enough to start nuclear reactions. The rate of those reactions, and thus how hot the star gets, depends on how much gas ends up in the ball. Blue Rigel is 20 times more massive than our yellow sun.

But what about Betelgeuse? It’s red, so does that mean it’s smaller than the Sun? Nope. There are two kinds of red stars: “adult” main sequence stars (which are the most common of all stars), and red giants in their final days. Betelgeuse has almost as much mass as Rigel. It is red because it is dying.

As stars use up their hydrogen fuel, the centers contract, and the outer layers expand out and cool. The stars become giant red puff balls regardless of what color they started out. In about 5 billion years, the Sun will become one of these red giants, expanding out past the orbit of Venus and toasting Earth’s marshmallow. It only took Betelgeuse about 10 million years to reach the giant, or in this case, supergiant, phase. Because of its huge mass, Rigel will become a red supergiant too, likely in the next few million years.

The red giant stage is a relatively short period of a star’s life, which is why there are so few visible in the sky. The red giant stage is followed by a final collapse of the center of the star as it runs out of fuel and can’t push back against gravity’s squeeze. For small and average stars, the collapse produces a white (hot) dwarf star about the size of Earth. Big stars like Betelgeuse and Rigel collapse violently, producing supernovas and leaving behind pulsars or black holes. Astronomers estimate that Betelgeuse’s supernova will outshine a full Moon when it happens: which could be tomorrow or a million years from now.

So while enjoying the colorful “preview” show of Orion this fall, have fun thinking about how this constellation will look when Betelgeuse “moons” the sky and Rigel blushes red!

Writing about Space

An excerpt of my memoir, A Passion for Space, describing my experiences as a flight controller during the first space shuttle launch, will be included in the FenCon 2018 Program Book this September. Attend to get your copy!

My next book, coauthored with Buzz Aldrin, To the Moon and Back: My Apollo 11 Adventure, a pop-up book from National Geographic with art by Bruce Foster, is available for preorder now from Amazon. Look for it in stores everywhere on October 16.

My science fact article, “In Defense of the Planet,” is in the Nov/Dec 2018 issue of Analog. Get your subscription now!

Speaking about Space

Teachers, librarians, and event organizers, please consider me for Author Visits. Writers and publishers, I offer science consulting, content and technical editing.

September 21-23, Science GOH at FenCon XV in Dallas. Writer GOH is Larry Niven. Tickets are available at the door.

September 29, Attending SCBWI Houston conference. Come and get a special pop-up book mark for To the Moon and Back designed by artist Bruce Foster.

October 2, Instructor for first class of Women and Space course at Rice University’s Glasscock School of Continuing Studies.

October 12, Featured speaker on Friday 11 to noon at the National Science Teachers Association conference in Reno, Nevada.

See my contact page for a complete appearance schedule and photos from previous events.