NGC 147 (also known as DDO3 or Caldwell 17) is a

dwarf spheroidal galaxy A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are comp ...

about 2.58

Mly 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 ...

away in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Cassiopeia. NGC 147 is a member of the Local group of galaxies and a

satellite galaxy A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents are bound ...

of the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

(M31). It forms a physical pair with the nearby galaxy

NGC 185 NGC 185 (also known as Caldwell 18) is a dwarf spheroidal galaxy located 2.08 million light-years from Earth, appearing in the constellation Cassiopeia. It is a member of the Local Group, and is a satellite of the Andromeda Galaxy (M31). NGC ...

, another remote satellite of M31. It was discovered by

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...

in September 1829. Visually it is both fainter and slightly larger than NGC 185 (and therefore has a considerably lower surface brightness). This means that NGC 147 is more difficult to see than NGC 185, which is visible in small

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

s. In the ''Webb Society Deep-Sky Observer's Handbook'', the visual appearance of NGC 147 is described as follows:

Large, quite faint, irregularly round; it brightens in the middle to a stellar nucleus.

The membership of NGC 147 in the Local Group was confirmed by

Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the United States from 1931 to 1959. Biography The son of a teacher, Baade finished school in 1912. He then studied maths, physics and astro ...

in 1944 when he was able to resolve the galaxy into individual stars with the telescope at Mount Wilson near

Los Angeles Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the largest city in the state of California and the second most populous city in the United States after New York City, as well as one of the world' ...

Characteristics

A survey of the brightest

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

(AGB) stars in the area of radius 2′ from the center of NGC 147 shows that the last significant star-forming activity in NGC 147 occurred around 3 Gyr ago. NGC 147 contains a large population of older stars which show a spread in

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

and age. The metallicity spread suggests that NGC 147 has had chemical enrichment. However, H I has not been observed and the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

(ISM) mass upper limit is much lower than expected had the material which is emitted from evolving stars been kept in the galaxy. This implies depletion of the ISM.

Distance measurements

At least two techniques have been used to measure distances to NGC 147. The surface brightness fluctuations distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to NGC 147 using this technique is 2.67 ± 0.18

(870 ± 60 kpc). However, NGC 147 is close enough that the

tip of the red giant branch Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red-giant-branch stars in a galaxy as a standard candle to gauge the distance to that galaxy. It has been used in conj ...

(TRGB) method may be used to estimate its distance. The estimated distance to NGC 147 using this technique is 2.21 ± 0.09 Mly (680 ± 30 kpc). Averaged together, these distance measurements give a distance estimate of 2.53 ± 0.11 Mly (780 ± 30 kpc).

Notes

average(870 ± 60, 680 ± 30) = ((870 + 680) / 2) ± ((60² + 30²)^0.5 / 2) = 780 ± 30

References

External links

SEDS – NGC 147
* {{DEFAULTSORT:NGC 0147 Dwarf spheroidal galaxies Local Group Andromeda Subgroup Cassiopeia (constellation) 0147 00326 002004 017b 18290908

Characteristics

Distance measurements

See also

Notes

References

External links