The Draco Dwarf is a spheroidal galaxy which was discovered by Albert George Wilson of Lowell Observatory in 1954 on photographic plates of the National Geographic Society's Palomar Observatory Sky Survey (POSS). It is part of the Local Group 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 Luminosity, luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents ...

of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy. The Draco Dwarf is situated in the direction of the Draco

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 first constellati ...

at 34.6° above the

galactic plane The galactic plane is the plane (geometry), 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 ''galac ...

Characteristics

Paul W. Hodge analyzed the distribution of its stars in 1964 and concluded that its ellipticity was 0.29 ± 0.04. Recent studies have indicated that the galaxy may potentially hold large amounts of

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

. 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 st ...

(RGB) stars; five carbon stars 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

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

RR Lyrae

In 1961, Walter Baade and Henrietta H. Swope 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. From this work a RR Lyrae derived distance modulus 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 (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 In probability theory and statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real number, real-valued random variable. The general form of its probability density function is f(x ...

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 of the values of a variable about its Expected value, mean. A low standard Deviation (statistics), deviation indicates that the values tend to be close to the mean ( ...

(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 proc ...

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 of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

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. 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. In 2024, a group of scientists using the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

measured proper motions of Draco with 18 years of data, making it the first

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

to have its 3D velocity dispersion profile radially resolved. The group of astronomers showed that Draco's

distribution is in better agreement with the LCDM model, helping to alleviate the cusp-core problem.

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
The quoted total mass is the cumulative dark matter mass (much higher than the luminous mass) at 900 pc from the galaxy's center.

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 ?