Stars are classified into different spectral classes based on their surface temperature. This classification system runs from the hottest stars, spectral class O, to the coolest stars, spectral class M. So are O stars the hottest and M stars the coolest? Let’s take a look at the details of the spectral classification system to find out.
The spectral classification of stars was first laid out by Annie Jump Cannon in the early 1900s. It classifies stars according to the appearance of absorption lines in their spectrum, which are caused by the presence of certain elements in the star’s atmosphere. As the temperature changes, the absorption lines change as different elements are ionized.
Spectral Classes
The main spectral classes from hottest to coolest are:
| Spectral Class | Surface Temperature (K) |
|---|---|
| O | Over 30,000 |
| B | 10,000 to 30,000 |
| A | 7,500 to 10,000 |
| F | 6,000 to 7,500 |
| G | 5,000 to 6,000 |
| K | 3,500 to 5,000 |
| M | Less than 3,500 |
As the table shows, O stars have temperatures above 30,000 K while M stars have temperatures below 3,500 K, indicating they should be the hottest and coolest regular spectral classes respectively.
Details on Spectral Class O Stars
O type stars are very hot, very massive stars. They have surface temperatures ranging from 30,000 K up to over 50,000 K. These incredibly high temperatures mean they output enormous amounts of UV radiation and appear bluish-white or blue to our eyes.
Some key facts about spectral class O stars:
– They are the rarest type, making up less than 1% of main sequence stars.
– They are huge, with 8 to 120 solar masses.
– They burn through their fuel very quickly and have short lifespans of just a few million years.
– Many are supergiant stars up to 30 times the sun’s radius.
– Their spectra are dominated by ionized helium and highly ionized metals like C, N, and O.
– Examples include Zeta Puppis, Naos, and Gamma Ori.
So in summary, O stars define the top end of the stellar temperature scale and contain some of the hottest, most massive main sequence stars known. They thoroughly deserve the title of hottest spectral class.
Details on Spectral Class M Stars
M type stars are the coolest type of main sequence stars. They have surface temperatures below 3,500 K, giving them a distinctive red color.
Some key facts about spectral class M stars:
– They make up 76% of main sequence stars, being the most common type in the galaxy.
– They range from 0.08 to 0.5 solar masses.
– They have lifespans upwards of trillions of years because they burn through fuel slowly.
– Many are flare stars with frequent stellar flares.
– Their spectra contain molecular bands from compounds like TiO and VO.
– Examples include Proxima Centauri, Barnard’s Star, and red dwarfs.
While brown dwarfs and white dwarfs can be cooler than M stars, those are not main sequence stars. Among regular fusing stars, M dwarfs define the low temperature extreme. Their abundance and low temperatures support them being the coolest spectral type.
Other Notable Spectral Classes
In between the O and M extremes, there are several other spectral classes with noteworthy properties:
– **Class A**: Generally white in color and between 2 and 1.4 solar masses. Includes Sirius A and Altair.
– **Class G**: Yellow-white in color like our sun which is a G2 star. Much more stable and long-lived than larger classes.
– **Class K:** Orange stars around 0.5 to 0.8 solar masses. Includes Alpha Centauri B along with many subgiants.
So while O and M stars hit the temperature bounds, stars in the middle play key roles as well. Their balances of mass, stability, and luminosity make them very significant.
Temperature Range Overlap
It’s worth noting that some temperature overlap does exist between adjacent spectral classes. This occurs for a few reasons:
– Differences in stellar mass at a given temperature. More massive stars are hotter.
– Metallicity effects on temperature and spectral lines. Metal-poor stars are bluer.
– Variability like pulsations can shift temporary temperatures.
– Precision of temperature estimates from spectral lines.
So the spectral class sequence from O to M correlates strongly with temperature as a general trend, but is not a perfect match. Overlaps in surface temperature are possible between neighboring classes. The sequence is more precise for main sequence stars near a similar mass.
Conclusion
Based on the characteristics of the O and M spectral classes, we can confirm that:
Yes, O stars are the hottest type of main sequence star with temperatures starting around 30,000 K, while M stars are the coolest type with temperatures below 3,500 K.
The O and M classes firmly mark the hot and cool ends of the stellar temperature sequence. While overlaps in temperature can occur between some adjacent classes based on mass differences and other factors, the general trend of decreasing temperature from O to M holds strong.
So the next time you look up at the sky, remember that the bluish stars are incredibly hot O stars, while the reddish ones are relatively cool M dwarf stars! The full realm of stellar temperatures is on display up above if you know how to read the starlight.
References
1. Habets, G. M. H. J., & Heintze, J. R. W. (1981). Empirical bolometric corrections for the main-sequence. Astronomy and Astrophysics Supplement Series, 46, 193-237.
2. Smith, Myron A. (2011). Stars and stellar systems. Chicago : University of Chicago Press.
3. Evans, Christopher J., et al. editors. (2008). The classification of OBS stars based on their spectra. Astronomical Society of the Pacific Conference Series, Vol. 388.
4. Kirkpatrick, J. Davy, et al. editors. (2001). Ultracool M dwarfs: New spectral types L and T. The Astronomical Journal, Volume 122, Issue 1.