The Sun is a as a G2V type star, a yellow dwarf and a main sequence star.
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| Zoom Astronomy The Stars |
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The Sun is a as a G2V type star, a yellow dwarf and a main sequence star. |
The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or absolute magnitude). On it, astronomers plot stars' color, temperature, luminosity, spectral type, and evolutionary stage. This diagram shows that there are 3 very different types of stars:
| Star Type | Color | Approximate Surface Temperature | Average Mass (The Sun = 1) | Average Radius (The Sun = 1) | Average Luminosity (The Sun = 1) | Main Characteristics | Examples |
|---|---|---|---|---|---|---|---|
| O | Blue | over 25,000 K | 60 | 15 | 1,400,000 | Singly ionized helium lines (H I) either in emission or absorption. Strong UV continuum. | 10 Lacertra |
| B | Blue | 11,000 - 25,000 K | 18 | 7 | 20,000 | Neutral helium lines (H II) in absorption. | Rigel Spica |
| A | Blue | 7,500 - 11,000 K | 3.2 | 2.5 | 80 | Hydrogen (H) lines strongest for A0 stars, decreasing for other A's. | Sirius, Vega |
| F | Blue to White | 6,000 - 7,500 K | 1.7 | 1.3 | 6 | Ca II absorption. Metallic lines become noticeable. | Canopus, Procyon |
| G | White to Yellow | 5,000 - 6,000 K | 1.1 | 1.1 | 1.2 | Absorption lines of neutral metallic atoms and ions (e.g. once-ionized calcium). | Sun, Capella |
| K | Orange to Red | 3,500 - 5,000 K | 0.8 | 0.9 | 0.4 | Metallic lines, some blue continuum. | Arcturus, Aldebaran |
| M | Red | under 3,500 K | 0.3 | 0.4 | 0.04 (very faint) |
Some molecular bands of titanium oxide. | Betelgeuse, Antares |
| TYPE | Star |
|---|---|
| Ia | Very luminous supergiants |
| Ib | Less luminous supergiants |
| II | Luminous giants |
| III | Giants |
| IV | Subgiants |
| V | Main sequence stars (dwarf stars) |
| VI | Subdwarf |
| VII | White Dwarf |
Yellow dwarfs are small, main sequence stars. The Sun is a yellow dwarf.
A red giant is a relatively old star whose diameter is about 100 times bigger than it was originally, and had become cooler (the surface temperature is under 6,500 K). They are frequently orange in color. Betelgeuse is a red giant. It is about 20 times as massive as the Sun about 14,000 times brighter than the Sun, and about 600 light-years from Earth.
A blue giant is a huge, very hot, blue star. It is a post-main sequence star that burns helium.
A double star is two stars that appear close to one another in the sky. Some are true binaries (two stars that revolve around one another); others just appear together from the Earth because they are both in the same line-of-sight.
A binary star is a system of two stars that rotate around a common center of mass (the barycenter). About half of all stars are in a group of at least two stars.
An eclipsing binary is two close stars that appear to be a single star varying in brightness. The variation in brightness is due to the stars periodically obscuring or enhancing one another. This binary star system is tilted (with respect ot us) so that its orbital plane is viewed from its edge.
Cepheid variables are stars that regularly pulsate in size and change in brightness. As the star increases in size, its brightness decreases; then, the reverse occurs. Cepheid Variables may not be permanently variable; the fluctuations may just be an unstable phase the star is going through. Polaris and Delta Cephei are examples of Cepheids.
| Some Mira Variable Stars | Magnitude Range | Period (days) |
|---|---|---|
| R Carinae | 3.9-10.5 | 308.7 |
| R Centauri | 5.3-11.8 | 546.2 |
| Mira (Omicron Ceti) | 3.4-9.3 | 332.0 |
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