Reflection nebula
Encyclopedia
In Astronomy
, reflection nebulae are clouds
of dust which are simply reflecting the light of a nearby star
or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula
, but is enough to give sufficient scattering
to make the dust visible. Thus, the frequency spectrum
shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized (Kaler, 1997). Edwin Hubble
distinguished between the emission and reflection nebulae in 1922.
Reflection nebulae are usually blue because the scattering
is more efficient for blue light than red (this is the same scattering process that gives us blue skies
and red sunset
s).
Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.
Some 500 reflection nebulae are known. Among the nicest of the reflection nebulae are those surrounding the stars of the Pleiades
. A blue reflection nebula can also be seen in the same area of the sky as the Trifid Nebula
. The giant star
Antares
, which is very red (spectral class
M1), is surrounded by a large, red reflection nebula.
Reflection nebulae may also be the site of star formation
.
In 1922, Hubble published the result of his investigations on bright nebulae. One part of this work is the Hubble luminosity law for reflection nebulae which make a relationship between the angular size (R) of the nebula and the apparent magnitude
(m) of the associated star:
where k is a constant that depends on the sensitivity of the measurement.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, reflection nebulae are clouds
Interstellar cloud
Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium. Depending on the density, size and temperature of a given cloud, the hydrogen in it...
of dust which are simply reflecting the light of a nearby star
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...
or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula
Emission nebula
An emission nebula is a cloud of ionized gas emitting light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star...
, but is enough to give sufficient scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
to make the dust visible. Thus, the frequency spectrum
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized (Kaler, 1997). Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...
distinguished between the emission and reflection nebulae in 1922.
Reflection nebulae are usually blue because the scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
is more efficient for blue light than red (this is the same scattering process that gives us blue skies
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...
and red sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...
s).
Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.
Some 500 reflection nebulae are known. Among the nicest of the reflection nebulae are those surrounding the stars of the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
. A blue reflection nebula can also be seen in the same area of the sky as the Trifid Nebula
Trifid Nebula
The Trifid Nebula is an H II region located in Sagittarius. Its name means 'divided into three lobes'...
. The giant star
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
Antares
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...
, which is very red (spectral class
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
M1), is surrounded by a large, red reflection nebula.
Reflection nebulae may also be the site of star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
.
In 1922, Hubble published the result of his investigations on bright nebulae. One part of this work is the Hubble luminosity law for reflection nebulae which make a relationship between the angular size (R) of the nebula and the apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
(m) of the associated star:
- 5 logLogarithmThe logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
(R) = -m + k
where k is a constant that depends on the sensitivity of the measurement.