Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1965 during a rocket flight and is one of the strongest X-ray sources detectable from Earth, producing a peak X-ray flux density of 2.3×10−23 W/(m2⋅Hz) (2.3×103 jansky). It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 21.2 times the mass of the Sun and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon has 300 km "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms.
A stellar black hole is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses. They are the remnants of supernova explosions, which may be observed as a type of gamma ray burst. These black holes are also referred to as collapsars.
26 Andromedae, abbreviated 26 And, is a binary star system in the constellation Andromeda. 26 Andromedae is the Flamsteed designation. It has a combined apparent visual magnitude of 6.10, which is near the lower limit of visibility to the naked eye. The distance to this system can be estimated from its annual parallax shift of 5.35 mas, which yields a distance of about 600 light years. At that distance, the visual magnitude of the stars is diminished from an extinction of 0.04 due to interstellar dust. The system is moving further from the Earth with a heliocentric radial velocity of +3.3 km/s.
Cygnus X-3 is a high-mass X-ray binary (HMXB), one of the stronger binary X-ray sources in the sky. It is often considered to be a microquasar, and it is believed to be a compact object in a binary system which is pulling in a stream of gas from an ordinary star companion. It is one of only two known HMXBs containing a Wolf–Rayet star. It is invisible visually, but can be observed at radio, infrared, X-ray, and gamma-ray wavelengths.
NGC 4639 is a barred spiral galaxy located in the equatorial constellation of Virgo. It was discovered by German-born astronomer William Herschel on April 12, 1784. John L. E. Dreyer described it as "pretty bright, small, extended, mottled but not resolved, 12th magnitude star 1 arcmin to southeast". This is a relatively nearby galaxy, lying approximately 72 million light-years away from the Milky Way. It is a companion to NGC 4654, and the two appear to have interacted roughly 500 million years ago. NGC 4639 is a member of the Virgo Cluster.
Gliese 412 is a pair of stars that share a common proper motion through space and are thought to form a binary star system. The pair have an angular separation of 31.4″ at a position angle of 126.1°. They are located 15.8 light-years distant from the Sun in the constellation Ursa Major. Both components are relatively dim red dwarf stars.
4U 1700-37 is one of the stronger binary X-ray sources in the sky, and is classified as a high-mass X-ray binary. It was discovered by the Uhuru satellite. The "4U" designation refers to the fourth Uhuru catalog.
HD 27245, also known as HR 1335 or rarely 25 H. Camelopardalis is a solitary red-hued star located in the northern circumpolar constellation Camelopardalis. It has an apparent magnitude of 5.4, making it faintly visible to the naked eye. Gaia DR3 Parallax measurements place it approximately 607 light years away from it the Solar System and is drifting further away with a heliocentric radial velocity of 25.2 km/s. At its current distance, HD 27245's brightness is diminished by 0.36 magnitudes due to extinction from interstellar dust. It has an absolute magnitude of −0.27.
V4641 Sagittarii is a variable X-ray binary star system in the constellation Sagittarius. It is the source of one of the fastest superluminal jets in the Milky Way galaxy.
V404 Cygni is a microquasar and a binary system in the constellation of Cygnus. It contains a black hole with a mass of about 9 M☉ and an early K giant star companion with a mass slightly smaller than the Sun. The star and the black hole orbit each other every 6.47129 days at fairly close range. Due to their proximity and the intense gravity of the black hole, the companion star loses mass to an accretion disk around the black hole and ultimately to the black hole itself.
HD 24479, also designated as HR 1204, is a solitary, bluish-white hued star located in the northern circumpolar constellation Camelopardalis. The star is visible to the naked eye with an apparent visual magnitude of 5.04. Based on Gaia DR3 parallax measurements, it is located 385 light years from the Sun. However, it is receding with a somewhat constrained heliocentric radial velocity of 4.6 km/s. At its current distance, HD 24479's brightness is diminished by 0.29 magnitudes due to interstellar dust.
23 Orionis is a double star located around 1,200 light-years away from the Sun in the equatorial constellation of Orion. It is visible to the naked eye as a dim, blue-white-hued point of light with a combined apparent visual magnitude of 4.99. The pair are moving away from the Earth with a heliocentric radial velocity of +18 km/s, and they are members of the Orion OB1 association, subgroup 1a.
Nu Horologii, Latinized from ν Horologii, is the Bayer designation for a single star in the southern constellation of Horologium. It was catalogued by the Dutch explorer Frederick de Houtman in 1603. With an apparent visual magnitude of 5.3, this star can be seen with the naked eye from the southern hemisphere. Based upon parallax, Nu Horologii lies at a distance of 169 light-years from Earth. It is drifting closer to the Sun with a radial velocity of +31 km/s.
BP Crucis is an X-ray binary system containing a blue hypergiant and a pulsar.
4U 1543-475 is a recurrent X-ray transient located in the southern constellation Lupus, the wolf. IL Lupi is its variable star designation. It has an apparent magnitude that fluctuates between 14.6 and 16.7, making it readily visible in large telescopes but not to the naked eye. The object is located relatively far at a distance of approximately 17,000 light years based on Gaia DR3 parallax measurements.
BP Tauri is a young T Tauri star in the constellation of Taurus about 416 light years away, belonging to the Taurus Molecular Cloud.
V723 Monocerotis is a variable star in the constellation Monoceros. It was proposed in 2021 to be a binary system including a lower mass gap black hole candidate nicknamed "The Unicorn". Located 1,500 light years from Earth, it would be the closest black hole to our planet, and among the smallest ever found.
Gaia BH1 is a binary system consisting of a G-type main-sequence star and a likely stellar-mass black hole, located about 1,560 light-years (478 pc) away from the Solar System in the constellation of Ophiuchus. As of May 2024, it is the nearest known system that astronomers are reasonably confident contains a black hole, followed by Gaia BH3, Gaia BH2 and A0620-00.
HD 10390 is a solitary star located in the northern constellation Triangulum. It is faintly visible to the naked eye as a bluish-white hued point of light with an apparent magnitude of 5.64. The object is located relatively close at a distance of 292 light-years based on Gaia DR3 parallax measurements and it is drifting closer with a heliocentric radial velocity of −1.9 km/s. At its current distance, HD 10390's brightness is diminished by an interstellar extinction of only five-hundredths of a magnitude and it has an absolute magnitude of +1.00.
