Another classification is based on mass. Starting with the least dense and progressing to the most dense, this is the sequence: Supergiant; Red Giant; Main Sequence ; Red Dwarf; Brown Dwarf; White Dwarf; Neutron Star; and Black Hole. This is also ranked according to (decreasing) size (diameter). The sequence just shown covers a mass range of about 1000. Few stars are larger than 100 times the mass of the Sun; few likewise are smaller than 1/10th of a solar mass. Very large stars burn up their atomic fuel rapidly and are not around more than a million years or so. Very small masses do not get hot enough to begin the Hydrogen fusion process that initiates a star's life.
Supergiants are dimensionally wide, and also luminous. NGC 3603 is the largest star in the Milky Way, being 116 times the mass of the Sun:
The brightest star in the northern hemisphere of the sky is Sirius, an A type star (see the H-R plots below and accompanying paragraphs which explain the letter designation of stars) of apparent magnitude -1.47 that lies 8.7 light years away. Here is how it appears through a telescope:
Closest to the Sun is the red dwarf Proxima Centauri, being 4.2 light years away. Just slightly farther away (4.4 l.y.) are Alpha Centauri A and B (visible in southern hemisphere), stars similar to the Sun that are among the brightest in the heavens.
Here is a telescope view of Alpha Centauri A:
The map below is a plot of the distances from Earth (outer circle is 13.1 light years in radius) of the 25 nearest individual or binary stars or local clusters in our region of the Milky Way Galaxy. Many of these stars are red dwarfs (see next page):
(Information Bonus: Just beyond this map's edge is the star Vega (27 light years away). It has two claims to fame: 1) it alternates with Polaris as the North Star used in navigation; the Earth's precession brings Vega into this position every 11000 years, and 2) It was the nearby star used as the host for an extraterrestrial civiliation in Carl Sagan's extraordinary science fiction novel "Contact" (later made into the movie of the same name "starring" Jodie Foster; in the film contact was made with a planet near Vega as a signal picked up by the Socorro, NM radio telescope array - as initially interpreted that signal consisted of a string of prime numbers [those divisible only by themselves and 1]).
Lets look farther out into our galactic neighborhood. Referring to the above diagram, the following is extracted verbatim from the caption accompanying this image that was displayed on the Astronomy Picture of the Day Website for February 17, 2002: "What surrounds the Sun in this neck of the Milky Way Galaxy? Our current best guess is depicted in the above map of the surrounding 1500 light years constructed from various observations and deductions. Currently, the Sun is passing through a Local Interstellar Cloud (LIC), shown in violet, which is flowing away from the Scorpius-Centaurus Association of young stars. The LIC resides in a low-density hole in the interstellar medium (ISM) called the Local Bubble, shown in black. Nearby, high-density molecular clouds including the Aquila Rift surround star forming regions, each shown in orange. The Gum Nebula, shown in green, is a region of hot ionized Hydrogen gas. Inside the Gum Nebula is the Vela Supernova Remnant, shown in pink, which is expanding to create fragmented shells of material like the LIC. Future observations should help astronomers discern more about the local Galactic Neighborhood and how it might have affected Earth's past climate."
The largest star so far measured in the Milky Way is Mu Cephi (in the galactic cloud IC1396), seen as the orange disc (also called Herschel's Garnet star) near top center of this HST image. Located about 1800 light years from Earth, it is almost 2500 times the diameter of the Sun.
This is an example of a rare type of star known as a Hypergiant (see next page). Another even bigger star (2800 times the solar diameter; 2.4 billion miles) is Epsilon Aurigae (in the constellation Auriga, the Charioteer), residing in the Milky Way about 3300 light years from Earth. This star, also known as Al Maaz (Arabic for he-goat) and visible to the naked eye) is considered by many astronomers to be the "strangest" star in the firmament. Every 27 years this star (magnitude 3.2) undergoes a diminishing of brightness (about 60000 times greater than the Sun) lasting about 2 years. The last such event was in 1983; the next in 2010. It is thus one of a class called "eclipsing stars". The cause of this regular pattern of luminosity change is still uncertain; some astronomers think it is caused by the passage of a second massive star across Epsilon Aurigae's face but that binary is so far undetected, leading to the hypothesis that the drop in luminosity occurs when a cloud of dark material (dust) orbiting the star as a clump obscures Epsilon Aurigae each time it moves through the line of sight to the Earth.
Most stars bigger than the Sun are not as huge as Mu Cephi or Epsilon Aurigae. The majority are no larger than about 100x the diameter of the Sun. This diagram illustrates the relative size of some common stars (setting the Sun's diameter as 1), which establishes our star as rather ordinary in the size scheme within the Milky Way:
One of the largest stars whose size can be accurately determined is VY Canis Majoris, in the Milky Way about 5000 l.y. from Earth. Best estimate of its diameter is about 2100 times that of the Sun.
VY Canis Majoris is a Hypergiant star. Here is the best telescope view of this massive stellar body.
This star is a superb example of how one view within one segment of the spectrum gives a specific impression of an apparently simple visualization but is misleading in that a different spectral region discloses a much different appearance. VY Canis Majoris is actually surrounding by a much larger cloud of gas of varying composition, as evident in this pair of HST images.
What is going on is that the star, because of its huge size, is destined to be short-lived and is already unstable, throwing off much of its mass. It is likely to be destroyed in less than another 100000 years as a supernova.
Another appropriate way to distinguish stars by size is to rank them according to mass. In the Milky Way, the Arches cluster contains the most massive single star (about 130 solar masses) found yet in the M.W.; at the time theorists thought this is a reasonable upper limit throughout the Universe:
But, a new record for "most massive" star was claimed in July 2010 by astronomers using the ESO telescope in South America. Star R136a1 lies within a young cluster of stars in the nearby Large Magellanic Cloud. It has 320 times the mass of the Sun and 10 million times the solar luminosity. This young (about 1 million years) star will be short-lived. It is shown here among other large stars:
More than half of the stars in a typical galaxy are also tied locally to a second star as a companion (the mutual interrelation of two stars is referred to by the term 'binary'), such that each of the pair orbits around a common center in space determined by their mass-dependent mutual gravitational attraction. This arrangement is exemplified by the image made by the HST Faint Object Camera (FOC) of the Persei 56 group:
Mizar, one of the stars making up the Big Dipper, was first observed by Galileo in 1650. He believed he saw two stars that seemed to revolve about each other. This is the first record of a true binary. Mizar A and Mizar B are shown here in a modern telescope view (each one is itself mated to a small second star, so that this is actually a paired set of binaries).
Some stars are grouped into more than one companion; ternary groupings (three stars orbiting about a common center of gravity) are fairly common. Here is an image of four stars orbiting as a unit about a gravity center in the galaxy M73.
Binary star systems are recognized by three means: 1) visual, through a telescope (as in the above two images); 2) by periodic drops in brightness caused by passage of one star across another (eclipse; an uncommon observation condition); and 3) by measuring spectral characteristics in which both a Doppler shift towards the red and the blue occur as one star moves away and the other towards Earth (and the reverse) along pathways of their mutual orbits.
To demonstrate the second means, examine this diagram which shows the brightness levels (and magnitude variations) for a binary star in which one is larger and brighter than the other (thus there are different decreases in brightness when the brighter star passes in front of the less luminous star, and vice versa). Incidentally, this method is also used to hunt for and verify planets associated with stars.
Spectral line shifts are used to study the motions of binary stars. We will treat stellar spectroscopy in detail on page 20-7 As a preview, the spectral method can be illustrated by looking at a pair of spectral strips for two similar stars that are mutually orbiting:
Bright lines for Hydrogen appear in the top and bottom (dark background) strips. This fixes a reference location for excited Hydrogen in the rest state. The two center spectral strips include the same Hydrogen lines, the first strip acquired from one and the second the other star. Note that the lines in one have moved to the left and the other to the right of the reference lines position. The spectrum on the bottom center has been blueshifted (see page 20-9) towards shorter wavelengths; the spectrum at the top center has been redshifted towards longer wavelengths. This is explained thusly: The bottom star is in motion towards the observing system on Earth whereas the top star is moving away from the telescope. This would occur when the two stars are aligned sideways to the line of sight and are moving in opposite directions around a common center of gravity. This diagram amplifies this explanation:
The Chandra X-ray Observatory has imaged a close binary pair in the M15 Galaxy. Prior to obtaining this image, the object was thought to be a single star, but at X-ray wavelengths, it is now resolved into a faint blue star and a nearby companion believed to be a neutron star giving off high energy radiation. Thus:
Most binary stars exist as two separated entities. But in 2008 an observation of two binaries that actually are enjoined (like Siamese twins) by shared solar matter were reported from a galaxy 13 million l.y. away. This is an artist's enhancement of one of these peanut-shaped pairs:
Turning now to stellar evolution, to preview what will be examined in some detail later on this page and the next page, the pattern of a star's history follows a pathway that, depending on its total mass, eventually splits into one of two branches (> or \>), as it leaves what is known as the Main Sequence. This is: 1) Development of a large cloud of denser gas made up of predominantly molecular Hydrogen (H2) + dust --> Protostar --> T-Tauri Phase --> Main Sequence (if mass less than 8 solar masses)--> Red Giant --> Planetary Nebula --> White Dwarf; or 2) (if mass greater than 8 solar masses) Main Sequence --\> Supernova --\> Neutron Star or Black Hole (if mass [size] is greater than 50-100 solar masses).
The star types are categorized into Spectral Classes which are defined on the basis of certain chemical elements that become excited at different temperatures and give off characteristic radiation at specific wavelengths. The classes are designated by the letters (O, B,...etc.) assigned to each group. Here are spectra for some of the different classes (this is treated in some detail on page 20-7).
Both star classification and evolution can be summarized in a graphlike chart that consists of a plot of luminosity (vertical axis) or, alternatively the related magnitude parameter, versus star surface temperature which is expressed also by (correlated with) the star's visual color (note also the Spectral Type designations at the top). This is known as the Hertzsprung-Russell (H-R) Diagram. (The masses of the stars in the diagram increase to the left on the abscissa; Red Giants are big but have low mass densities [many less than the Sun, since mass was lost in evolving to that state].) Most known stars lie along the Main Sequence; they describe a stage in which a protostar reaches some fixed size and mass and commences burning of most of its Hydrogen before changing to some other star type off the sequence. Here is a H-R diagram:
The Sun (also called Sol) is a G type star on the Main Sequence. Very hot stars on the M.S. include the Blue White stars. Red Dwarfs are M stars. Large luminous but low temperature stars form several Giant classes. Small, still luminous and very hot (at surface) stars make up the White Dwarfs.
The above H-R plot also shows along the right ordinate the relative sizes of each star compared with the Sun. As far as we now know, stars do not completely vanish, but survive as dwarfs or Black Holes (but the latter in principle can disappear by evaporation as Hawking radiation).
Among the off-Main Sequence evolved star groups are four types of Giants (Sub; Red; Bright; Super), T Tauri. These are discussed again on this page or elsewhere in this Section. Not shown among the Dwarfs is the recent designation of LT for Brown Dwarfs. Note that the letters at the bottom include some like B0 and B5 or K0-K5; this denotes subdivision of each class into temperature subclasses (0 being hottest and 5 coolest in a class). Temperature ranges (in °K) are: O class = greater than 30000; B = 11000 - 30000; A = 7500 - 11000; F = 6000 - 7500; G = 5000 - 6000; K = 3500 - 5000; M = less than 2500. Colorwise, the first three are all "blue-white" stars, F is bluish to white; G is white to yellow; K is yellow orange; and M is red. Although not directly shown in an H-R diagram, there is a systematic increase in mass of a star going from the right to the left end of the plot.
A star on the Main Sequence will follow some pathway during its subsequent history. To illustrate this progression, look first at this evolution diagram for a star the mass of the Sun. The first diagram extends the history of a F star by showing the sequence of star stages from its very inception as a nebular mass that grows into a protostar, then to the M.S., next, as it burns most of its , off the M.S. as a Red Giant, followed by an explosion, and subsequent evolution into a final dwarf state.:
This second diagram follows the history of a G star (which is the path to be followed by the Sun in about 5 billion years) after it leaves the Main Sequence:
The key steps in the progression are 1) exhaustion of the main nuclear fuel; 2) change to a Red Giant, with shedding of some mass; 3) explosion to the Planetary Nebula phase, dispersing much of the star's mass into interstellar space; 4) survival of a central core as a White Dwarf star.
The pathways of protostars to the Main Sequence depend on their mass (in multiples of a solar mass) at the stage when they commence proceeding to the M.S and initiate Hydrogen fusion. The times involved in this transition will vary systematically with mass; thus, a 15 solar mass protostar takes only about 10000 years to reach the M.S. whereas a 2 solar mass star may require up to 10,000,000 years for the process to begin fusion:
This next diagram shows the evolutionary history of three stars of differing mass at the upper, central, and lower ends of the Main Sequence after they leave the M.S.:
These pathways are somewhat generalized: A Sun-sized star (Class G) eventually becomes a Red Giant and then a White Dwarf. A smaller star (Class M) can evolve directly into a White Dwarf. A much larger star (Class B or O) will destroy itself as a Supernova that yields a planetary nebula (the gaseous remnants from the explosion; page 20-2a) but may retain some of its mass as a Neutron Star.