The Draco Dwarf is a spheroidal galaxy which was discovered by

Albert George Wilson Albert George Wilson (July 28, 1918 – August 27, 2012) was an American astronomer and a discoverer of minor planets. He was born in Houston, Texas. He received his Ph.D. in mathematics from Caltech in 1947; his thesis title was ''Axially ...

Lowell Observatory Lowell Observatory is an astronomical observatory in Flagstaff, Arizona, United States. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark ...

in 1954 on photographic plates of the National Geographic Society's

Palomar Observatory Sky Survey Palomar may refer to: Places * Any of several locations in San Diego County, California: ** Palomar Mountain ** Palomar Observatory, located on Palomar Mountain ** Palomar College in San Marcos, California ** Palomar Medical Center in Escondido, C ...

(POSS). It is part of the

Local Group The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

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

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. The Draco Dwarf is situated in the direction of the

Draco Draco is the Latin word for serpent or dragon. Draco or Drako may also refer to: People * Draco (lawgiver) (from Greek: Δράκων; 7th century BC), the first lawgiver of ancient Athens, Greece, from whom the term ''draconian'' is derived * ...

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 ...

at 34.6° above the

galactic plane The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usuall ...

Characteristics

Paul W. Hodge analyzed the distribution of its stars in 1964 and concluded that its

ellipticity Flattening is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening is ...

was 0.29 ± 0.04. Recent studies have indicated that the galaxy may potentially hold large amounts of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

. Having an absolute magnitude of -8.6 and a total luminosity of only , it is one of the faintest companions to our Milky Way. Draco Dwarf contains many

red giant branch The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...

(RGB) stars; five

carbon star A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes mos ...

s have been identified in Draco Dwarf and four likely

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 have been detected. The Draco Dwarf is estimated to be 80 ± 10 kpc from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

and span a distance of 830 ± 100 × 570 ± 70 pc.

RR Lyrae

In 1961,

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 ...

and

Henrietta H. Swope Henrietta Hill Swope (October 26, 1902 – November 24, 1980) was an American astronomer who studied variable stars. In particular, she measured the period-luminosity relation for Cepheid stars, which are bright variable stars whose periods of va ...

studied Draco Dwarf and discovered over 260 variables, of the 138 in the cluster's center, all but five were determined to be

RR Lyrae variables RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and ...

. From this work a RR Lyrae derived

distance modulus The distance modulus is a way of expressing distances that is often used in astronomy. It describes distances on a logarithmic scale based on the astronomical magnitude system. Definition The distance modulus \mu=m-M is the difference between the ...

of 19.55 is found which implies a distance of 81 kpc.

Metallicity

The Draco Dwarf contains primarily an old population of stars and insignificant amounts of

interstellar matter 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 interstellar s ...

(being basically dust free). From 75% to 90% of its stars formed more than ~10 Gyr ago followed by a low rate of formation with a small burst of star formation around 2-3 Gyr ago. It has a single Gaussian distribution with average metallicity of e/H= −1.74 dex with a

standard deviation In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...

(sigma/σ) of 0.24 dex and a small tail of metal-rich stars. The central region of Draco Dwarf exhibits a concentration of more metal-rich stars there being more centrally concentrated red

horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) ...

stars than blue horizontal branch stars.

Dark matter

Recently, dwarf spheroidal galaxies have become key objects for the study of

. The Draco Dwarf is one which has received specific attention.

Radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

computations of Draco have revealed a large internal velocity dispersion giving a mass to luminosity ratio of up to /, suggesting large amounts of dark matter. It has been hypothesized that large velocity dispersions could be explained as ''tidal dwarfs'' (virtually unbound stellar streams from dwarf galaxies tidally disrupted in the Milky Way potential). However, Draco Dwarf's narrow

width does not support this model. This only leaves the dark matter explanation and makes Draco Dwarf the most dark matter dominated object known as of 2007. The dark matter distribution within Draco Dwarf is at least nearly

isothermal In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a ...

. At large radii, radial velocity dispersion exhibit strange behavior. One possible explanation for this would be the presence of more than one stellar population. This suggests the need for further study of Draco Dwarf population's metallicity and ages and of dwarf spheroidals in general.

Notes

Assuming an absolute magnitude of +0.5 V for RR Lyrae the apparent modulus of the Draco Dwarf is 19.58 m - M. Using a reddening value towards Draco Dwarf of 0.03 ± 0.01 we get a true distance modulus of 19.55.
Using the distance modulus formula of 1 we get an RR Lyrae estimated distance of 81 kpc.
Apparent Magnitude of 10.9 - distance modulus of 19.52 (80 kpc) = −8.6
distance 80 ± 10 kpc × tan( diameter_angle = 35′.5 × 24′.5 ) = 830 ± 100 × 570 ± 70 pc diameter

References

External links

* {{Sky, 17, 20, 12.4, +, 57, 54, 55, 260000 Dwarf spheroidal galaxies Peculiar galaxies Local Group Milky Way Subgroup Draco (constellation) 10822 60095 ?