Friday Night Science: What Kind Of Star Is Our Sun?

What do we mean when we say our Sun is a G2V star? 

The Sun's neither yellow nor orange and by no means red. It's all due to Earth's atmosphere
 scattering the sunlight. Remove the atmosphere, and there's the Sun
— (in true color) a white main-sequence G2V star. 
Credit: Public Domain, via Wikimedia Commons. 

Stars are classified according to their spectra. Just as we humans have unique fingerprints and dogs have unrepeated nose prints, every single star out there carries a distinctive spectral signature or star prints if you fancy the term. The science of spectroscopy is simple. If you hold out a prism to the Sun’s rays, the white light reveals its true identity as a continuous (rainbow) spectrum of colors — the VIBGYOR spectrum. Voila! You’ve obtained the solar spectrum. To be more specific, you’re only looking at the continuous spectrum of the Sun. However, if you guide a very narrow beam of sunlight through an instrument known as a spectrometer, an untold number of dark lines appear upon the continuous spectrum. 

The science of spectroscopy is simple enough to be explained by this schematic only. At the same time, its elegance lies in the fact that just by looking at a star's spectra, specifically, absorption spectrum — dark lines against the familiar rainbow background allows us to decode the star's deepest secrets. 

 Credit: NASA, ESA, Leah Hustak [STScI].

The typical solar spectrum — absorption spectrum — contains thousands of dark lines, some placed very close to each other while some may be apart. These dark lines are better known as Fraunhofer lines after Joseph von Fraunhofer, who rose to fame for accidentally stumbling upon the characteristic spectrum of the Sun in 181a. He was, however, not the first to do so. Twelve years ago, William Hyde Wollaston became the first to witness these never-before-seen lines. Fraunhofer was quick to realize that the dark lines couldn’t be an artifact of the prism or his spectrometer but must belong to the Sun itself. And not just for the Sun. Immediately, it followed that all stars in the night sky exhibit a unique barcode. 

Joseph von Fraunhofer's original etching of the solar spectrum with the eponymous Fraunhofer lines. 
The curve above depicts the peak sensitivity of the human eye to daytime illumination — another brilliant achievement of Fraunhofer. A normal eye is most sensitive at 555 nm. 
Credit: Public Domain, via Wikimedia Commons (link). 

But why does the Sun and, for that matter, any star have these lines in the first place? Below the visible photosphere and deeper still, where 600 million tons of hydrogen is converted into 596 million tons of helium each second, while the rest becomes pure electromagnetic energy, the Sun shines as a perfect black body. A black body is that source of light that emits a continuous rainbow spectrum; hold out a prism before a candle flame or an incandescent (filament) bulb, and you’ll see what a black body is. The Sun’s atmosphere encompasses a different chemistry than its interior and an assortment of elements from silicon, carbon, nitrogen, magnesium, calcium, and others. As sunlight travels through the Sun’s atmosphere, these neutral elements, i.e., their constituent electrons, absorb electromagnetic energy that shows up as a series of dark lines across the otherwise continuous spectrum. 

A star’s unique barcode encodes all that there is to know about a star. Apart from giving us knowledge about the star’s chemical makeup — how much hydrogen, helium, oxygen, nitrogen, iron, silicon, etc., the prevalence and strength of the spectral lines allow us to estimate how hot the star is at the (visible) surface — photosphere — is. Secondly, the relative abundance of the elements tells us how old the star might be and at what stage of stellar evolution it might be. Further analysis of stellar spectra can also reveal whether a particular star is moving away from us or closing in — known as the Doppler effect after Christian Doppler, and from an endless list of things, the series of dark dashes also permits us to speculate about the possibility of alien life across the galaxy. 

One among many, here's the Harvard system of spectral classification. 
Credit: Public Domain, via Wikimedia Commons (link). 

In the early days of stellar spectroscopy, stars were classified alphabetically on the strength of their hydrogen lines. The most common white to bluish-white A-type stars, slightly more massive than our Sun, such as Vega, Deneb, Fomalhaut, and Sirius, have very prominent hydrogen (absorption) lines. Massive to very-massive B-type stars such as Rigel, Alnilam, and Upsilon Orionis of the Orion constellation have moderate hydrogen lines. Although both G- and O-type stars have very weak hydrogen lines, they are fundamentally different. The latter kind are the most massive stars (> 16 times the mass of our Sun) in the known universe. Stellar spectroscopy becoming mainstream, astronomers realized that hydrogen lines don’t serve as a good marker for classifying stars. And so, for reasons we won’t be going into, the alphabetical order of stars got jumbled into a confusing O-B-A-F-G-K-M sequence or the mnemonic, ‘Oh Be A Fine Girl, Kiss Me’. 

Fraunhofer lines of the solar spectrum. The extended solar spectrum has about
twenty-five thousand lines between 295 nm (extreme ultraviolet) to 1000 nm (near infrared).  
Credit: Public Domain, via Wikimedia Commons (link). 

Our Sun, a G-type star, has prominent absorption lines of calcium metal or calling out its particular ionization state — doublet Ca II at wavelengths 396.8 nm and 393.4 nm labeled as the H and K lines, respectively. At 486.1 nm, the solar spectrum spots the characteristic hydrogen-beta (H-β) line — the cyan-blue emission H-β shows up here as the dark F line. The same happens for the brilliant red hydrogen-alpha (H-⍺) line at 656.3 nm — labeled the C line. Similar to calcium, strong sodium lines — the characteristic D₁-D₂ — materialize at  589.6 nm and 589.0 nm, respectively, marked by D. Strong lines of neutral oxygen come around at 759.4 nm — marked by A and at 686.7 nm — marked by B. Interestingly, the A and B oxygen lines don’t come from the oxygen in the Sun but in Earth’s atmosphere. On this note, it's worth mentioning that helium was not discovered on Earth but in the spectra of the Sun. 

Other Fraunhofer lines include multiple absorption lines of oxygen, helium, mercury, iron, magnesium, strontium, tin, aluminum, sulfur, neon, silicon, etc. It's worth mentioning that not all lines form in the same region of the Sun's atmosphere. As mentioned earlier, while the absorption lines happen close to the visible photosphere, higher up, 3,000 - 5,0000 km in the hotter chromosphere where temperatures range from 6,000-20,000 K, the dark lines are replaced by strong emission lines of hydrogen and helium. 

In summary, our Sun's spectral tag G2V translates as follows: G2 stands for the second hottest G-type star whose surface temperature is about 5,800 K, has weak hydrogen lines but strong lines of calcium, sodium, and some other metals, and finally, the V means it's not a colossal giant nor a puny one but a bright dwarf. And it's not the only one of its kind, for there are 21 G-type stars within the solar neighborhood of 10 pc (parsecs) or 32.6 light-years.