Cassiopeia Dwarf

{{short description|Galaxy in the constellation Cassiopeia}}

{{redirect|Andromeda 7|the star|7 Andromedae}}

{{Infobox Galaxy

| name = Cassiopeia Dwarf

| image = Cassiopeia Dwarf (PGC 2807155) Hubble WikiSky.jpg

| image_size = 250px

| caption =Cassiopeia Dwarf by HST/WikiSky

| epoch = J2000

| type = dSph

| ra = {{RA|23|26|31.0}}{{cite web

| title=NASA/IPAC Extragalactic Database

| work=Results for Cassiopeia Dwarf

| url=http://nedwww.ipac.caltech.edu/

| access-date=2006-11-30 }}

| dec = {{DEC|+50|41|31}}

| dist_ly = {{cvt|2.45|Mly|Mpc|lk=on}}{{cite journal|doi=10.3847/1538-4357/abdec1|title=The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies. IV. The Star Formation History of Andromeda VII Derived from Long-period Variable Stars|year=2021|last1=Navabi|first1=Mahdieh|last2=Saremi|first2=Elham|last3=Javadi|first3=Atefeh|last4=Noori|first4=Majedeh|last5=Van Loon|first5=Jacco Th.|last6=Khosroshahi|first6=Habib G.|last7=McDonald|first7=Iain|last8=Alizadeh|first8=Mina|last9=Danesh|first9=Arash|last10=Gozaliasl|first10=Ghassem|last11=Molaeinezhad|first11=Alireza|last12=Parto|first12=Tahere|last13=Raouf|first13=Mojtaba|journal=The Astrophysical Journal|volume=910|issue=2|page=127|arxiv=2101.09900|bibcode=2021ApJ...910..127N|s2cid=231698580 |doi-access=free }}

| z = {{nowrap|−307 ± 2 km/s}}

| appmag_v = 12.9

| absmag_v = −13.3 ± 0.3

| size_v = {{nowrap|2.5{{prime}} × 2.0{{prime}}}}

| constellation name = Cassiopeia

| notes = satellite galaxy of the Andromeda Galaxy

| names = Andromeda VII, And VII, Cas dSph, PGC 2807155,

{{nowrap|Cassiopeia Dwarf Galaxy}}{{cite simbad

| title=NAME Andromeda VII

| access-date=2010-03-06 }}

}}

The Cassiopeia Dwarf (also known as Andromeda VII) is a dwarf spheroidal galaxy about 2.45 Mly away in the constellation Cassiopeia. The Cassiopeia Dwarf is part of the Local Group and a satellite galaxy of the Andromeda Galaxy (M31). In the sky, it appears behind the Milky Way's galactic plane, and so it is reddened by 0.194 magnitudes. With a luminosity of {{solar luminosity|{{val|1.8e7}}|link=y}} and a stellar mass of {{solar mass|{{val|19.73e6}}|link=y}}, it is the brightest and most massive of the Andromeda Galaxy's dwarf spheroidal galaxy satellites. It also has the highest metallicity out of all of them.

The Cassiopeia Dwarf was found in 1998, together with the Pegasus Dwarf, by a team of astronomers (Karachentsev and Karachentseva){{Cite journal

| bibcode=2005AJ....129.2232P

| title=The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda I and Andromeda III

| journal=The Astronomical Journal |date=May 2005

| volume=129 | issue=5 | pages=2232–2256 | doi=10.1086/428372

|author1=Pritzl, Barton J. |author2=Armandroff, Taft E. |author3=Jacoby, George H. |author4=Da Costa, G. S. |arxiv = astro-ph/0501083 | s2cid=9749493

}} in Russia and Ukraine. The Cassiopeia Dwarf and the Pegasus Dwarf are farther from M31 than its other known companion galaxies, yet still appear bound to it by gravity. Neither galaxy contains any young, massive stars or shows traces of recent star formation. Instead, both seem dominated by very old stars, with ages of up to 10 billion years. The main period of star formation occurred around 6.2 billion years ago, and 90% of the star formation in the Cassiopeia Dwarf was done by 5 billion years ago.