Young Astronomers Blog, Volume 28, Number 2.
Astronomers measure the magnitude of a star in a bit of a strange way that dates to Hipparchus. A negative magnitude is brighter than a positive magnitude. A difference of one magnitude is actually a 2.5 times difference in brightness. A star’s apparent magnitude is the perceived magnitude when we view the star from the Earth. A star’s absolute magnitude is its apparent magnitude at a distance of 10 parsecs (32.6 light-years). Stars are also assigned a classification (Morgan-Keenan) of O for the hottest, B, A, F, G, K, down to M for the coolest. Remember this by the mnemonic “Oh Be A Fine Girl/Guy Kiss Me.”
In the early part of the twentieth century, Ejnar Hertzsprung and Henry Norris Russell discovered that stars could be represented in what is now called the Hertzsprung-Russell (H-R) diagram. The horizontal axis shows the stellar classification/surface temperature in descending temperature/spectral class. The vertical axis shows the luminosity (as compared to the Sun)/absolute magnitude. Stars, for much of their life, are arranged in a diagonal from the top left to the bottom right called the main sequence. Large/hot blue giants are toward the upper left. Small/cool red dwarfs are toward the bottom right. Our Sun is in the middle.
For most of a star’s life, the force of gravity trying to collapse the star is balanced with the nuclear force at the center of the star where hydrogen is being converted into helium. As such, a star will remain in its position on the main sequence throughout much of its lifetime as it burns hydrogen. When the hydrogen runs out, the core contracts and the outer shell expands and cools, and a red supergiant is born. The star moves toward the middle to upper right of the H-R diagram. Here a star will remain as it burns heavier elements in its core and hydrogen in its shell. Once these elements are exhausted, the star’s ultimate fate depends on its size.
Low mass stars will collapse into a white dwarf, and the star will move to the left middle of the H-R diagram. The outer layers are expelled, creating what we call a planetary nebula. A white dwarf star can also pull matter from a companion star and, if so, it will eventually explode as a type IA supernova.
More massive stars will eventually produce iron and the process stops. The core collapses and the temperatures reach billions of degrees. The iron atoms are crushed together. The repulsive force of the nuclei overcomes gravity and the core recoils. The shock wave is what we see as a type II supernova. During the recoil, material is heated and heavier elements (e.g., silver, gold and uranium) are formed. Stars with a core around 1 ½ to 3 times the mass of the Sun, will collapse until the nuclear force balances gravity and a neutron star is formed. For stars with a core greater than 3 times the mass of the Sun, the nuclear force is overcome by gravity and a black hole is formed.
The star Betelgeuse, a type M star in Orion’s right shoulder, is in its red giant phase. If it were located at the center of our solar system, it would extend out to the planet Jupiter. Astronomers have recently noticed that Betelgeuse is dimming, which indicates something might be happening. We do expect Betelgeuse to explode as a supernova. But the real question is when will it explode? Betelgeuse is a few hundred light years away, so it might have already exploded, but the light from the explosion has yet to reach us. So, keep looking up, you might see something spectacular in the constellation Orion, or you might not. Only time will tell.
Selected Sources and Further Reading
“Magnitude (astronomy) facts for kids.” Kiddle. (accessed January 23, 2020). https://kids.kiddle.co/Magnitude_(astronomy)
Alan MacRobert. “The Stellar Magnitude System.” Sky & Telescope. August 1, 2006. https://www.skyandtelescope.com/astronomy-resources/the-stellar-magnitude-system/
“Hertzsprung-Russell Diagram.” Cosmos Swinburne Center for Astrophysics and Supercomputing, Swinburne University of Technology. (accessed December 29, 2019). http://astronomy.swin.edu.au/cosmos/H/Hertzsprung-Russell+Diagram
Petersen, Carolyn Collins. “Diagramming the Lives of Stars.” ThoughtCo, Nov. 6, 2019, thoughtco.com/hertzsprung-russell-diagram-4134689. https://www.thoughtco.com/hertzsprung-russell-diagram-4134689
“The Morgan-Keenan System.” StarParty.com. (accessed December 29, 2019). https://starparty.com/topics/astronomy/stars/the-morgan-keenan-system/
“Stars.” NASA Share the Science. (accessed December 29, 2019). https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve
“Stellar Evolution – The Birth, Life, and Death of a Star.” NASA Student Features. (accessed December 29, 2019). https://www.nasa.gov/audience/forstudents/9-12/features/stellar_evol_feat_912.html
“The Life Cycles of Stars: How Supernovae Are Formed.” GSFC/NASA Imagine the Universe. (accessed December 29, 2019). https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-lifecycles.html
Andrew Zimmerman Jones “Why Do Stars Burn and What Happens When They Die?” ThoughtCo. March 29, 2019: https://www.thoughtco.com/why-stars-burn-and-star-death-2698853
Erika K. Carlson. “Betelgeuse’s bizarre dimming has astronomers scratching their heads.” Astronomy. December 27, 2019. http://www.astronomy.com/news/2019/12/betelgueses-bizarre-dimming-has-astronomers-scratching-their-heads
Bob King. “Betelgeuse is Dimming . . . Why?” Sky & Telescope. December 31, 2019. https://www.skyandtelescope.com/observing/fainting-betelgeuse/
Daniel Brown. “Betelgeuse: Star’s weird dimming sparks rumors that its death is imminent.” Phys.org. January 14, 2020. https://phys.org/news/2020-01-betelgeuse-star-weird-dimming-rumors.html
Evan Gough. “Betelgeuse is Continuing to Dim! It’s Down to 1.506 Magnitude. Universe Today. January 22, 2020. https://www.universetoday.com/144694/betelgeuse-is-continuing-to-dim-its-down-to-1-506-magnitude/
Technical Reading
Hertzsprung, E. “On the Use of Photographic Effective Wavelengths for the Determination of Color Equivalents”. Publications of the Astrophysical Observatory in Potsdam. 1. 22 (63). 1911.
Russell, Henry Norris. “Relations Between the Spectra and Other Characteristics of the Stars”. Popular Astronomy. 22: 275–294. May 1914.
E. F. Guinan et al. “The Fainting of the Nearby Red Supergiant Betelgeuse.” The Astronomer’s Telegram. December 8, 2019. http://www.astronomerstelegram.org/?read=13341
Edward F. Guinan and Richard J. Wasatonic. “The Continued Unprecedented Fading of Betelgeuse.” The Astronomer’s Telegram. January 23, 2020. http://www.astronomerstelegram.org/?read=13410