Star counts are bookkeeping surveys of
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s and the
statistical
Statistics (from German: ''Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industria ...
and
geometrical
Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ca ...
methods used to correct the survey data for
bias
Bias is a disproportionate weight ''in favor of'' or ''against'' an idea or thing, usually in a way that is closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group, ...
. The surveys are most often made of nearby stars in the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
galaxy
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
.
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, actually bluish white) through white (type F) to 'red' (type M, actually ruddy orange). Types L and T are used for
brown dwarfs
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most m ...
, whose 'colors' are in the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
.
* Size: Size is usually designated by Roman numerals I (
supergiants
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars ...
) 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
The thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. It is the main non-centre (e.g. galactic bulge) density, of such matter. That of the Milky Way is thought to have a scale height of around i ...
'', ''thick disk'', ''central bulge'', and ''halo''.
* Multiplicity: Most stars are members of double star, or triple star, or even double-double star systems. Our own sun appears to be unusual 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
In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass ...
brightness than they do in distance.
* Our line of sight through the Milky Way is interrupted by great clouds of gas and dust, which block our view of stars more than a few thousand
light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s away.
* The
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
is located in the disk of the Milky Way, in the northern edge of the ''thin disk'' and on the inner edge of a
spiral arm
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''[Orion–Cygnus Arm
The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion ...]
. 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
A giant is a being of human appearance, sometimes of prodigious size and strength, common in folklore.
Giant(s) or The Giant(s) may also refer to:
Mythology and religion
*Giants (Greek mythology)
*Jötunn, a Germanic term often translated as 'gi ...
and blue
dwarfs) are the most common stars listed in general
star catalog
A star catalogue is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, ...
s, even though on average they are rare in space. Small dim stars (
red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
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 supergiant
A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They have luminosity class I and spectral class B9 or earlier.
Blue supergiants are found towards the top left of the Hertzsprung–Russell diagram, above an ...
ζ Puppis is 400 million times more
luminous than the nearest star, a
red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
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). ''ζ Puppis'' is one of the brightest of the visible 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.
{{DEFAULTSORT:Star Count
Count
Count (feminine: countess) is a historical title of nobility in certain European countries, varying in relative status, generally of middling rank in the hierarchy of nobility. Pine, L. G. ''Titles: How the King Became His Majesty''. New York: ...