Stephenson 2, also known as RSGC2 (''Red Supergiant Cluster 2''), is a young massive

open cluster An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

belonging to 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. It was discovered in 1990 as a cluster of red supergiants in a photographic, deep infrared survey by the astronomer Charles Bruce Stephenson, after whom the cluster is named. It is located in the constellation Scutum at the distance of about 6 kpc from the Sun. It is likely situated at the intersection of the northern end of the Long Bar of the Milky Way and the inner portion of the

Scutum–Centaurus Arm The Scutum–Centaurus Arm, also known as Scutum-Crux arm, is a long, diffuse curving streamer of stars, gas and dust that spirals outward from the proximate end of the Milky Way's central bar. The Milky Way has been posited since the 1950s to h ...

—one of the two major spiral arms.

Observation history

Distance estimates

When the cluster was originally discovered in 1990, Stephenson 2 was originally estimated to have a distance of around , much further than the cluster is thought to reside today. This greater distance was calculated by the assumption that the cluster stars were all M-type

supergiants Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range ...

, then calculating the distance modulus based on their typical absolute magnitudes. In 2001, Nakaya et al. estimated the distance of the stars in the cluster to be 1.5 kiloparsecs (4,900 light-years), which is significantly closer than any other distance estimate given for the star and the cluster. Alternatively, a study around a similar timeframe gave a further distance of roughly 5.9 kiloparsecs (19,000 light-years). A study in 2007 determined a kinematic distance of kiloparsecs ( light-years) from comparison with the cluster's

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 ...

, considerably closer than the original distance of 30 kiloparsecs (98,000 light-years) quoted by Stephenson (1990). This value was later adopted in a recent study of the cluster. A similar kinematic distance of 5.5 kiloparsecs (18,000 light-years) was reported in a 2010 study, derived from the average radial velocity of four of the cluster's members (96 kilometers per second) and from an association with a clump of stars near Stephenson 2, Stephenson 2 SW, locating it near the

of the

. This value was later adopted in a 2012 study, which used the aforementioned distance to calculate the luminosities of the members, however it is noted that the uncertainty in the distance was greater than 50%. Despite this, it is also stated that distances to massive star clusters will be improved in the future. Verheyen et al. (2013) used the average radial velocity of the cluster (+109.3 ± 0.7 kilometers per second) to derive a kinematic distance of roughly 6 kiloparsecs (20,000 light-years) for the cluster.

Description

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelg ...

s have been confirmed as members of the cluster, far more than any other known cluster, both in and out of the Milky Way. This includes stars such as Stephenson 2 DFK 1, Stephenson 2 DFK 2, and Stephenson 2 DFK 49. A more recent study has identified around 80 red supergiants in the line of sight of Stephenson 2, approximately 40 of them with radial velocities consistent with being cluster members. However these stars are spread over a wider area than a typical cluster, indicating an extended

stellar association A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more visible stars. An association is primarily identified by commonalities in i ...

similar to that found around the nearby cluster

RSGC3 RSGC3 (''Red Supergiant Cluster 3'') is a young massive open cluster belonging to the Milky Way galaxy. It was discovered in 2010 in the GLIMPSE survey data. The cluster is located in the constellation Scutum at the distance of about 7 k ...

. The age of Stephenson 2 is estimated at 14–20 million years. The observed red supergiants with the mass of about 12–16

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es are

type II supernova A Type II supernova or SNII (plural: ''supernovae'') results from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun () to undergo this type ...

progenitors. The cluster is heavily obscured and has not been detected in the

visible light Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...

. It lies close to other groupings of red supergiants known as RSGC1,

, Alicante 7,

Alicante 8 Alicante 8, also known as RSGC4, (''Red Supergiant Cluster 4'') is an asterism, formerly thought to be a young massive open cluster, belonging to the Milky Way galaxy. It was discovered in 2010 in the 2MASS survey data. As of 2010, the only memb ...

, and Alicante 10. The mass of the open cluster is estimated at 30–50 thousand solar masses, which makes it the second most massive open cluster in the Galaxy.

Stephenson 2 SW

Some of the stars in the vicinity of the cluster lie in a loose grouping near the cluster, including Stephenson 2 DFK 1, Stephenson 2 DFK 49 and Stephenson 2-26. This grouping was first mentioned in Deguchi (2010) and was named Stephenson 2 SW because it lies south-west of the main cluster. While the radial velocities of its members are somewhat different from the main cluster's radial velocity,(by about 7.7 km/s) The difference between the 2 velocities is still relatively small, and not enough to rule out its association with the main cluster. Thus, it was assumed that it is possibly related to Stephenson 2 itself.

Members

Stars whose rows are colored in yellow are stars supposed to be part of Stephenson 2 SW.

References