Spectral classification (Temperature and color)

• In astronomy, stars are classified using several different systems, each based on distinct physical properties like temperature, intrinsic brightness, evolutionary stage, or chemical composition.
• The most common system is the Morgan-Keenan (MK) Spectral Classification. It groups stars based on their surface temperature, which directly dictates their color and the specific absorption lines visible in their light spectra.
• Each temperature range is known as a spectral type or class. From hottest to coolest the order is: (hotter) O B A F G K M (cooler)
  • Our Sun has a surface temperature of about 6,000 degrees C and is therefore designated as a G star.
  • This video summarizes the OBAFGKM classification system.
• Each spectral type (OBAFGKM) is divided into 10 subclasses, designated with a number, 0-9. The spectral types and sub-classes represent a temperature sequence, from hotter (O stars) to cooler (M stars), and from hotter (subclass 0) to cooler (subclass 9). So, for example, G0 is hotter than G1.
• Note there can be some confusion regarding the various letters. For example, the bluish star Sirius, the brightest star in the night sky, is actually a binary system, consisting of a bright Sirius A (the one we see) paired with a dim Sirius B (not visible without a powerful telescope). These letters (A and B) are not related to the spectral type and are simply used to distinguish the two stars. By coincidence, Sirius A happens to belong to spectral type A and Sirius B belongs to the spectral type B. Proxima Centauri is a three-star system (Proxima Centauri A, Proxima Centauri B, Proxima Centauri C) but these letters once again have nothing to do with their respective spectral types.
• Different star classes are characterized by the prominence of certain spectral lines.
• There are several mnemonics to remember this sequence
  • Oh Be A Fine Girl/Guy Kiss Me (This is the most famous one)
  • Only Boys Accepting Feminism Get Kissed Meaningfully
  • Old Baboons Angrily Fling Green Kiwis and Mangos
  • Only Bumbling Astronomers Forget Generally Known Mnemonics
  • Only Bad Astronomers Feel Good Knowing Mnemonics

Luminosity Classification (Size & Evolutionary Stage)

• Two stars can have the exact same temperature (spectral type) but vastly differ in size and energy output (e.g., red supergiant vs red dwarf). The MK (temperature) system accounts for this by adding a Roman numeral to designate the luminosity class, which relates to the star's density and radius. The two labels together unambiguously describe a star and its evolutionary state.
• For example, a giant (class III) is more evolved than a main-sequence star (class V).
• More examples:
  • The full classification for our Sun is G2 V. The G2 spectral type means it is yellow-white in color and the luminosity class V means it a hydrogen-burning, main-sequence star.
  • Betelgeuse is an M2 or a red supergiant.
  • Proxima Centauri is an M5 V, similar in color and surface temperature to the supergiant Betelgeuse, but less evolved and far dimmer because of its much smaller size.
• This video discusses the various luminosity types of stars, from dim red dwarfs to very luminous supergiants.

Metallicity

• If we look at the raw bulk ingredients, stars look nearly identical. Almost every active star in the universe is roughly 98-99% hydrogen and helium by mass.
• The 1-2% heavier elements is known in astronomy as metallicity. (Astronomers refer to elements heavier than hydrogen and helium as metals.)
• Metallicity turns out to be one of the most powerful tools we have for decoding the history of the universe.
• The universe started with only hydrogen, helium, and a trace of lithium. Every single heavier element (carbon, oxygen, iron, gold, etc.) was forged later inside the cores of stars or during supernovae. When a star forms, it traps a pristine sample of the interstellar gas cloud it was born in. Because the universe gets progressively more 'polluted' with heavy elements over time, a star's metallicity tells us when and where it was born:
  • Low metallicity (metal-poor): The star is ancient (Population II). It was born in the early universe before many supernovae had occurred.
  • High metallicity (metal-rich): The star is young (Population I, like our Sun). It was born recently from a gas cloud that had been enriched by generations of dying stars.
• By classifying stars this way, we can map out how our galaxy was built over 13 billion years.
• Metallicity can be determined spectroscopically.
  • Low-Metallicity Stars: The spectrum looks incredibly "clean" or empty. The continuum light flows through almost uninterrupted, and the metal absorption lines are incredibly faint, shallow, or completely absent.
  • High-Metallicity Stars: The spectrum is crowded with deep, dark absorption lines. In extreme cases, there are so many metal lines in the blue and ultraviolet parts of the spectrum that they blend together, physically blocking a massive amount of the star's light. This phenomenon is called line blanketing. It alters the shape of the continuum, absorbing blue light and re-radiating it in the red/infrared.
• Metallicity not only affects a star's spectrum but also its size, temperature, and evolution.
• If two stars have exactly same mass, the one with higher metallicity will be larger in size (radius) than the one with lower metallicity.
  • The reason for this is that the heavier elements are better at trapping light energy inside the star, which results in higher internal pressure and ultimately more expansion.
• The star with the higher metallicity will also be cooler and redder due to the expansion.
• This video discusses metallicity in stars and its relationship to the formation of Earth-like planets.

Hertzsprung-Russell (HR) Diagram

• When stars are plotted on a luminosity vs surface temperature diagram (HR diagram), several interesting patterns emerge:
  • Most stars fall on the Main Sequence.
  • On the Main Sequence, the more massive stars are bigger, hotter, more luminous, and die faster.
  • The life span of stars ranges from about 10 million years for the blue giants to about 100 billion years for the red dwarfs.
  • The most common type of star is the red dwarf (lower right); the least common type is the blue giant (upper left).
• This classification was originally proposed in 1912 by astronomers Ejnar Hurtzprung and Henry Norris Russell, hence the designation HR diagram.
• Luminosity of stars if often expressed in units of the Sun's luminosity (L = 3.9 x 10²⁶ Joules/s).
• The HR diagram spans a rather large range in luminosity, from 10^-4L on the low end to as much as 10⁶L on the high end.
• This video discusses the HR diagram and and the relationship between a star's temperature and its luminosity.
• This interactive applet might help you visualize some of the properties of the HR diagram.

Star size

• Of the 12 brightest stars in our sky, most are giants and supergiants.
• Our Sun is a main-sequence star dwarfed by a supergiant like Betelgeuse.
• Star mass ranges from 0.08xM_sun to 100xM_sun:
  • Stars more massive than about 100xM_sun release too much energy through nuclear fusion and are unstable.
  • Stars less massive than 0.08M_sun are too small to sustain nuclear fusion. Very large objects below this limit are sometimes called brown dwarfs.
     

Stellar Lifetimes

• The lifetime of a star is directly proportional to the amount of fuel it has (i.e., mass) and inversely proportional by the rate at which it burns the fuel (i.e., luminosity). Putting these together, we can estimate the lifetime t to be proportional to M/L.
• Empirically, for stars on the main sequence, luminosity is roughly propotional to the cube of the mass (L ~ M^3). Consequently, plugging this in for L, we find that the lifetime is inversely proportional to a power of the mass (t~1/M^2.5).
• Since the most massive stars are about 100 times the mass of the Sun, their lifetimes must be about 1,000 times shorter, or about 10^6 (10^10/10^4)years. This is indeed what we see; the most masive stars burn out in about 4 million years.
• Similarly, the least massive stars are about one tenth of a solar mass. They survive about 100 times longer than our Sun, or about 10^12 years. Because this is well over ten times the present age of the universe, none of these smaller stars have died yet.

Clusters

• Many stars are found in one of two types of clusters: open and globular.
  • A famous star cluster visible to the naked eye is the Pleiades, also known as the Seven Sisters.
  • Our Solar System is not part of a star cluster.
• The HR diagram is generated through a careful study of star clusters. Clusters are important because:
  • All the stars in a cluster lie at about the same distance from Earth.
  • All the stars in a cluster formed at about the same time.
• The age of a cluster corresponds to the main sequence turnoff point.
  • Stars with life spans equal to this age are exiting the main sequence.
  • Smaller stars with life spans longer than this age are still on the main sequence.
  • Larger stars with life spans shorter than this age are off the main sequence (dead).
• Open clusters are or are characterized by:
  • Few stars (10-1000)
  • Relatively small (about 30 ly)
  • Always found in the galactic disk
  • Relatively young (less than 7 BY)
  • Enriched in the heavy elements
• Globular clusters are or are characterized by:
  • Lots of stars (10⁴-10⁶)
  • Relatively large (~50-150 ly)
  • Found mostly in halo
  • Relatively old (12-16 BY)
  • Virtually no heavy elements

A little history

• In the early part of the 20th century, a classification scheme was devised for stars based on their spectra. The scheme was originally based only on the relative strengths of Hydrogen lines in the stars' atmosphere. Type A stars had the strongest hydrogen lines, type O the weakest. The different classes were then rearranged in order of decreasing surface temperature. Some letters were rejected (e.g., C, D, E) due to redundancy.
• The original system based on the strength of hydrogen lines was flawed because two stars with the same line strength could actually be two very different stars, with very different temperatures, as can be seen in this diagram.
• This video discusses stellar classification from a historical perspective.