Star count
Encyclopedia
Star Counts are bookkeeping surveys of star
s and the statistical
and geometrical
methods used to correct the survey data for bias. The surveys are most often made of nearby stars in the Milky Way Galaxy.
One of the interests of astronomy is to determine how many stars there are of each of several types that stars can be categorized into, and how these stars are distributed in space.
There are many finer subdivisions in all of the above categories.
Knowing that these effects create bias, astronomers analyzing star counts attempt to find how much bias each effect has caused and then compensate for it as well as they can.
Heavy, bright stars (both giants and blue dwarfs
) are the most common stars listed in generally star catalogs, even though on average they are obviously rare in space. Small dim stars (red dwarf
s) seem to be the most common stars in space, at least locally, but can only be seen with large telescopes, and then only when they are within a few tens of light-years from Earth.
For example, the blue giant
ζ Puppis
is 400 million times more luminous
than the nearest star, a red dwarf
named Proxima
, or α Centauri
C. Even though Proxima is only 4.2 light-years away from us, it is so dim that it cannot be seen with the naked eye (one of its companions, α Centauri A, is visible). The star ζ Puppis is one of the brightest of the visible extreme blue supergiants. It is so bright that it appears to be a second magnitude star, even though ζ Puppis is 1,399 light-years away.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s and the statistical
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....
and geometrical
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
methods used to correct the survey data for bias. The surveys are most often made of nearby stars in the Milky Way Galaxy.
One of the interests of astronomy is to determine how many stars there are of each of several types that stars can be categorized into, and how these stars are distributed in space.
Reasons for Star Counts
When performing star counts, astronomers consider many different categories that have been created to classify a few stars that have been well studied. One of the hopes of studying the results of star counts is to discover new categories. Different counts typically seek to categorize stars for only a few of the qualities listed below, and determine how common each considered quality is and how stars of that kind are distributed.- Temperature. In astronomy, temperature is usually shown using the letter codes O B A F G K M running from blue (type O) through white (type F) to red (type M). Types L and T are used for brown dwarfs, whose 'colors' are in the infraredInfraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
. - Size. Size is usually designated by Roman numerals I (supergiants) through V (dwarfs).
- Age. Stars are usually grouped into Population I (young) and Population II (old).
- Location. In the Milky Way Galaxy the groups are described as thin disk, thick disk, central bulge, and halo.
- Multiplicity. Most stars appear to be members of double star or triple star, or even double-double star systems. Our own sun appears to be rare for not having a companion star.
There are many finer subdivisions in all of the above categories.
Bias
There are many unavoidable problems in counting stars for the purpose of getting an accurate picture of the distribution of stars in space. The effects of our point of view in the galaxy, the obscuring clouds of gas and dust in the galaxy, and especially the extreme range of inherent brightness, create a biased view of stars.- Stars vary far more in intrinsic brightness than they do in distance.
- Our line of sight through the Milky Way Galaxy is interrupted by great clouds of gas and dust, which block our view of stars more than a few thousand light-yearLight-yearA light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away. - The SunSunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
is located in the disk of the Milky Way Galaxy, in the northern edge of the thin disk and on the inner edge of a spiral arm. There is good reason to believe that stars in the galaxy's thin disk are different from thicker part of the disk, and from the bulge and the halo. Some stars are obviously more common in spiral arms than in the disk in between the arms.
Knowing that these effects create bias, astronomers analyzing star counts attempt to find how much bias each effect has caused and then compensate for it as well as they can.
Inherent Luminosity Complications
The greatest problem biasing star counts is the extreme differences in inherent brightness of different sizes.Heavy, bright stars (both giants and blue dwarfs
Dwarf star
The term dwarf star refers to a variety of distinct classes of stars.* Dwarf star alone generally refers to any main sequence star, a star of luminosity class V.** Red dwarfs are low-mass main sequence stars....
) are the most common stars listed in generally star catalogs, even though on average they are obviously rare in space. Small dim stars (red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
s) seem to be the most common stars in space, at least locally, but can only be seen with large telescopes, and then only when they are within a few tens of light-years from Earth.
For example, the blue giant
Blue giant
In astronomy, a blue giant is a star with a spectral type of O or B and a luminosity class of III...
ζ Puppis
Zeta Puppis
Zeta Puppis is a star in the constellation of Puppis. It is also known by the traditional names Naos and Suhail Hadar in Arabic....
is 400 million times more luminous
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
than the nearest star, a red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
named Proxima
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
, or α Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
C. Even though Proxima is only 4.2 light-years away from us, it is so dim that it cannot be seen with the naked eye (one of its companions, α Centauri A, is visible). The star ζ Puppis is one of the brightest of the visible extreme blue supergiants. It is so bright that it appears to be a second magnitude star, even though ζ Puppis is 1,399 light-years away.