![]() Credit: Stocktrek Images/Tomasz Dabrowski / Getty Images Artist's impression of a star like our Sun swelling into a red giant and engulfing its orbiting planets. The star swells up and becomes a red giant or supergiant. The main sequence is not a sharply defined line on the HR diagram, though, as several other properties, like chemical composition and age, influence the location of a star on the diagram.Ī main sequence star is in a state of hydrostatic equilibrium – the internal pressure created by the nuclear reactions at the star’s core balances the inward pull of gravity, and the star is stable.īut once hydrogen burning in the core ceases, the star loses this stability and evolves away from, or off, the main sequence. Stars spend the majority of their lives on the main sequence the exact duration depends on the mass of the star.Īnd the main sequence is the most prominent curve on this diagram, forming a slanted ‘S’. This diagram is a graph of stars’ absolute magnitudes versus their surface temperatures. Stars in this phase lie on what we call the main sequence of the Hertzsprung-Russell (HR) diagram. The ‘main sequence’ part of the name refers to the fact that the Sun is at a stage in its life when the primary fuel source is the conversion of hydrogen to helium. A very small star could remain on the main sequence for tens to hundreds of billions of years-far longer than the current age of our Universe.The Sun is classified as a G-type main sequence star. These stars “live fast and die young!” A very large star may only be on the main sequence for 10 million years. Large stars burn through their supply of hydrogen very quickly. Bigger stars produce more energy, so their surfaces are hotter. The surface temperature of most stars is due to their size. ![]() The blue star in the lower right is the Class B star Rigel. The red supergiant Betelgeuse is seen near Orion’s belt. There are two types of Class M stars: red dwarfs and red giants.Īn artist’s depiction of a red dwarf star. Our Sun: the most important G class star in the Universe, at least for humans.Īrcturus is a Class K star that looks like the Sun but is much larger. ![]() What we see in the night sky as the single star “Polaris,” we also know as the North Star. There are two F class stars in this image, the supergiant Polaris A and Polaris B. The dot on the right, Sirius B, is a white dwarf. Sirius A is the brightest star that we see in the night sky. When you try to remember the order, you can use this phrase: “Oh Be A Fine Good Kid, Man.” ClassĪn artist’s depiction of the O class star Zeta Puppis.Ĭomputer generated image of Rigel, a Class B star. It wasn't until later that their order was corrected to go by increasing temperature. The letters don’t match the color names because stars were first grouped as A through O. Each class of star is given a letter, a color, and a range of temperatures. The most common way of classifying main sequence stars is by color ( Table below).
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