What is the closest star to us that is most like our Sun?

Alpha Centauri A. How do we get this answer? First, you have to know what kind of star our Sun is. The Sun is a medium-sized yellow star, with a surface temperature of about 5800 K ('K' stands for 'Kelvin,' a temperature scale. 0 K = -273 degrees Celsius), and the core temperature is 15.5 million K. The Sun is about 109 times bigger than the Earth; about one million earths could fit inside the sun!

In the world of astronomy, the Sun is called a G2 V star. What does this mean? Well, the first part, "G2," refers to the Sun's spectral type. A star is generally classified according to how its spectrum appears [The rainbow is a familiar spectrum of visible light]. A close look at a star's spectrum reveals spectral lines. Every chemical element - hydrogen, helium, neon, etc. - has a unique set of spectral lines (which appear upon magnification of a spectrum). So, by looking at a star's spectrum, its chemical composition can be determined.

Each spectral class is assigned a letter of the alphabet - OBAFGKM. A star's spectrum is closely related to its surface temperature, so the classification shows a temperature sequence as well. A G star, like the Sun, has a temperature in the range of 5,000 K to 6,000 K and has a spectrum showing prominent lines of hydrogen and ionized calcium (there are other features as well, including iron lines). Each spectral class is further divided into steps from 0 to 9, with 0 being the coolest subclass and 9 being the hottest subclass.

This table shows some of the spectral characteristics of stars, and how they're related to temperature and color:

Note: 1) here, "metals" are elements heavier than helium. 2) most star colors are pale, which is characteristic for hot, glowing objects.

Solar Spectrum

Credit & Copyright: Nigel Sharp (NOAO), FTS, KPNO, AURA, NSF

Well, that takes care of the "G2" part. What about "V"? In this case, "V" is the Roman numeral for "5," and it is an indication of how the Sun's luminosity (or brightness) affects spectral lines. Luminosity class V, means that the Sun is a main sequence star - it is burning hydrogen into helium. Luminosity classes range from I (supergiant stars) to V (main sequence stars).

So for stars like our Sun, we want to look for other G2 V stars.

How do we determine what stars are close to us? The parallax method is one way of determining distances to nearby stars. Parallax is when a nearby object appears to shift with respect to a far away object, when you move from one place to another. For example, if you hold your finger out away from you, close one eye and line your finger up with something father away, your finger will appear to shift if you move your head from side to side.

Observing the stars around us, we find that the nearest star system is a triple star system at 4.3 light years away. A light year is the distance light travels in a year: 9.46 trillion kilometers (5.9 trillion miles). In this system, is a G2V star - Alpha Centauri A - just what we're looking for. Alpha Centauri B is an orange K0 star, while Alpha Centauri C (otherwise known as Proxima Centauri) is a small M5 star.

Further reading and images on the Web about:

The Classification of Stellar Spectra
OBAFGKM:
http://www.noao.edu/image_gallery/html/im0649.html

Spectra and What Scientists can Learn from Them:
http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html

The 26 Nearest Stars