Stars Algol (from the Arabic
رأس الغول ''Ra's al-Ghul
, which means The Demon's Head
), also known by its Bayer designation Beta Persei, is the best-known star in Perseus. Representing the head of the Gorgon Medusa in Greek mythology, it was called Horus in Egyptian mythology and Rosh ha Satan'' ("
Satan's Head") in Hebrew. Located 92.8
light-years from Earth, it varies in
apparent magnitude from a minimum of 3.5 to a maximum of 2.3 over a period of 2.867 days. The star system is the prototype of a group of eclipsing
binary stars named
Algol variables, though it has a third member to make up what is actually a triple
star system. The brightest component is a
blue-white main-sequence star of
spectral type B8V, which is 3.5 times as massive and 180 times as luminous as the
Sun. and has 4.5 times the luminosity and 80% of its mass. which is located on average 2.69 AU from the other two stars.
AG Persei is another Algol variable in Perseus, whose primary component is a B-type main sequence star with an apparent magnitude of 6.69.
Phi Persei (Dajiangjunbei) is a double star, although the two components do not eclipse each other. The primary star is a
Be star of spectral type B0.5, possibly a giant star, and the secondary companion is likely a stellar remnant. The secondary has a similar spectral type to O-type
subdwarfs. With the historical name Mirfak (Arabic for
elbow) or Algenib, Alpha Persei is the brightest star of this constellation with an apparent magnitude of 1.79. A
supergiant of spectral type F5Ib located around 590 light-years away from Earth, Mirfak has 5,000 times the
luminosity and 42 times the diameter of the Sun. It is the brightest member of the
Alpha Persei Cluster (also known as
Melotte 20 and
Collinder 39), which is an open cluster containing many luminous stars. Neighboring bright stars that are members include the 3.0 magnitude Be stars
Delta (Sarvvis)
Psi (4.3), and
48 Persei (4.0);, the 2.9 magnitude
Beta Cephei variable Epsilon Persei (Áldu) and the stars
29 (5.2),
30 (5.5),
31 (5.0), and
34 Persei (4.7). Of magnitude 4.05, nearby
Iota Persei has been considered a member of the group, but is actually located a mere 34 light-years distant. This star is very similar to the Sun, shining with 2.2 times its luminosity. It is a yellow main sequence star of spectral type G0V. Extensive searches have failed to find evidence of it having a planetary system.
Zeta Persei is the third-brightest star in the constellation at magnitude 2.86. Around 750 light-years from Earth, it is a blue-white supergiant 26–27 times the radius of the Sun and 47,000 times its luminosity. It is the brightest star (as seen from Earth) of a
moving group of bright blue-white giant and supergiant stars, the Perseus OB2 Association or Zeta Persei Association. Zeta is a triple star system, with a companion blue-white main sequence star of spectral type B8 and apparent magnitude 9.16 around 3,900 AU distant from the primary, and a white main sequence star of magnitude 9.90 and spectral type A2, some 50,000 AU away, that may or may not be gravitationally bound to the other two.
X Persei is a double system in this association; one component is a hot, bright star and the other is a
neutron star. With an apparent magnitude of 6.72, it is too dim to be seen with the naked eye even in perfectly dark conditions. The system is an
X-ray source and the primary star appears to be undergoing substantial mass loss. Once thought to be a member of the Perseus OB2 Association,
Omicron Persei (Atik) is a multiple star system with a combined visual magnitude of 3.85. It is composed of two blue-white stars—a giant of spectral class B1.5 and main sequence star of B3—which orbit each other every 4.5 days and are distorted into ovoids due to their small separation. The system has a third star about which little is known. At an estimated distance of 1,475 light-years from Earth, the system is now thought to lie too far from the center of the Zeta Persei group to belong to it.
GRO J0422+32 (V518 Persei) is another X-ray binary in Perseus. One component is a
red dwarf star of spectral type M4.5V, which orbits a mysterious dense and heavy object—possibly a
black hole—every 5.1 hours. The system is an X-ray nova, meaning that it experiences periodic outbursts in the X-ray band of the
electromagnetic spectrum. If the system does indeed contain a black hole, it would be the smallest one ever recorded. Further analysis in 2012 calculated a mass of 2.1 solar masses, which raises questions as to what the object actually is as it appears to be too small to be a black hole.
GK Persei, also known as Nova Persei 1901, is a bright nova that appeared halfway between Algol and Delta Persei. Discovered on 21 February 1901 by Scottish amateur astronomer
Thomas David Anderson, it peaked at magnitude 0.2—almost as bright as
Capella and
Vega. It faded to
magnitude 13 around 30 years after its peak brightness.
Xi Persei, traditionally known as Menkhib, a blue giant of spectral type O7III, is one of the hottest bright stars in the sky, with a surface temperature of 37,500 K. It is one of the more massive stars, being between 26 and 32 solar masses, and is 330,000 times as luminous as the Sun. Named Gorgonea Tertia,
Rho Persei varies in brightness like Algol, but is a pulsating rather than eclipsing star. At an advanced stage of stellar evolution, it is a red giant that has expanded for the second time to have a radius around 150 times that of the Sun. Its helium has been fused into heavier elements and its core is composed of carbon and oxygen. It is a
semiregular variable star of the
Mu Cephei type, whose apparent magnitude varies between 3.3 and 4.0 with periods of 50, 120 and 250 days. The Double Cluster contains three even larger stars, each over 700
solar radii:
S,
RS, and
SU Persei are all semiregular pulsating
M-type supergiants. The stars are not visible to the naked eye; SU Persei, the brightest of the three, has an apparent magnitude of 7.9
DY Persei is a variable star that is the prototype of
DY Persei variables, which are carbon-rich
R Coronae Borealis variables that exhibit the variability of
asymptotic giant branch stars. This has been inferred to be an object with a maximum mass of 6 times that of
Jupiter and an orbital radius of 3.3 AU.
Deep-sky objects The
galactic plane of the
Milky Way passes through Perseus, but is much less obvious than elsewhere in the sky as it is mostly obscured by
molecular clouds. The
Perseus Arm is a spiral arm of the Milky Way galaxy and stretches across the sky from the constellation Cassiopeia through Perseus and Auriga to
Gemini and
Monoceros. This segment is towards the rim of the galaxy. Sometimes known as h and Chi (χ) Persei, respectively, they are easily visible through binoculars and small telescopes. Both lie more than 7,000 light-years from Earth and are several hundred light-years apart. Both clusters are of approximately magnitude 4 and 0.5 degrees in diameter. The two are
Trumpler class I 3 r clusters, though NGC 869 is a
Shapley class f and NGC 884 is a Shapley class e cluster. These classifications indicate that they are both quite rich (dense); NGC 869 is the richer of the pair. The clusters are both distinct from the surrounding star field and are clearly concentrated at their centers. The constituent stars, numbering over 100 in each cluster, range widely in brightness.
M34 is an open cluster that appears at magnitude 5.5, and is approximately 1,500 light-years from Earth. It contains about 100 stars scattered over a field of view larger than that of the full moon. and contains many stars with
circumstellar disks. Many
brown dwarfs have been discovered in this cluster due to its age; since brown dwarfs cool as they age, it is easier to find them in younger clusters. There are many nebulae in Perseus.
M76 is a
planetary nebula, also called the Little Dumbbell Nebula. It appears two
arc-minutes by one arc-minute across and has an apparent brightness of magnitude 10.1. It is very difficult to observe visually because its low surface brightness makes it appear dimmer than most other emission nebulae.
NGC 1333 is a
reflection nebula and a
star-forming region. Perseus also contains a giant
molecular cloud, called the
Perseus molecular cloud; it belongs to the
Orion Spur and is known for its low rate of star formation compared to similar clouds. Perseus contains some notable galaxies.
NGC 1023 is a
barred spiral galaxy of magnitude 10.35, around from Earth. The September Epsilon Perseids, discovered in 2012, are a meteor shower with an unknown parent body in the
Oort cloud. ==See also==