Northern Cross Asterism – Facts
The Northern Cross is a prominent asterism in the constellation of Cygnus (the Swan), located in the northern hemisphere. It is often confused with the whole constellation of Cygnus since it is made up of the brightest stars of the constellation — Deneb, Sadr, Aljanah, Fawaris and Albireo.
The asterism is easy to locate in the night sky, especially during the summer months, because it is high overhead for observers in the northern latitudes. It can also be seen during spring in the early morning to the East.
The Northern Cross asterism lies in the direction of the Milky Way’s disk, and the galactic plane (equator) runs through the asterism. It never dips below the horizon at or above 45° north latitude. From the southern hemisphere, it appears upside down and low in the sky during the winter months.
The star Deneb is the bright star that marks the Swan’s tail. Once located, it is easy to see the rest of the Northern Cross asterism. Deneb is also one of the three stars of the Summer Triangle, along with the even brighter stars Altair and Vega. The Northern Cross can be found within the Summer Triangle.
Deneb marks one end of the “Great Rift” in the Milky Way, a series of dark clouds that stretch all the way to the centre of our galaxy in Sagittarius constellation in the southern sky.
Stars Of The Northern Cross Asterism
Deneb, also known as Alpha Cygni, is the brightest star in Cygnus and the 19th brightest star in the night sky. It is a blue-white supergiant belonging to the spectral class A2 Ia and has an apparent magnitude of 1.25. The star has an an absolute magnitude of -7.0, making it one of the most luminous stars known. It is almost 60,000 times more luminous than our Sun and has around 20 times the mass. It is also one of the largest white stars known.
The name Deneb comes from the Arabic dhaneb, meaning “tail,” from the phrase Dhanab ad-Dajājah, which means “the tail of the hen.” The star is approximately 1,400 light years away from us and, on Mars, is the North Pole star. Together with the stars Altair in the constellation Aquila and Vega in Lyra, Deneb forms the Summer Triangle, which is a prominent asterism in the summer sky.
The star has stopped fusing hydrogen in its core and is expected to explode as a supernova within the next few million years. It also serves as a prototype for a class of variable stars known as the Alpha Cygni variables.
Sadr, also known as Gamma Cygni, is located at the intersection of the Northern Cross and belongs to the spectral class F8 lab, indicating that it is a supergiant. It has an apparent magnitude of 2.23 and is one of the brightest stars in the night sky.
The name Sadr comes from the Arabic word for “the chest,” şadr. It is also sometimes known by its Latin name, Pectus Gallinae, which means “the hen’s chest”. The star is located 1,800 light years away from Earth and is thought to be only about 12 million years old.
Sadr has twelve solar masses and a radius that is 150 times that of the Sun. It consumes its nuclear fuel more rapidly because of its mass. It is also surrounded by a diffuse emission nebula, IC 1318.
Aljanah, also known as Epsilon Cygni, is an orange giant star of the spectral type K0 III. This star has an apparent magnitude of 2.480. It has a radius 11 times that of the Sun and is 62 times more luminous than the Sun. It is also 72.7 light years away from us.
Aljanah used to share the name Gienah with Gamma Corvi in the constellation Corvus. However, the International Astronomical Union (IAU) approved the name Aljanah for the star in 2017. Gamma Corvi has retained the name Gienah. Both of these names come from the Arabic word janāħ. The word means “the wing”, because both stars mark the wings of the birds in their constellation.
Fawaris, also known as Delta Cygni, is a triple star that will take over as the North Star for at least 400 years around the year 11,250. It consists of two stars lying close together and a third star located a bit further from the main pair. Together, the system has an apparent magnitude of 2.87 and is located around 165 light years away from us.
The brightest component in the star system is a blue-white giant belonging to the spectral class B9 III. It is a fast rotating star with a speed of 135 kilometres per second and is thought to be approaching the final stages of life on the main sequence. The closer companion to the brightest component is a yellow-white star belonging to the spectral class F1 V. It has an apparent magnitude of 6.33. The third component is an orange giant of the 12th magnitude.
Albireo, also known as Beta Cygni, is the fifth brightest star in the constellation of Cygnus. It is located around 380 light years away from us and appears as a single third magnitude star to the naked eye. Its position marks the head of the swan in the constellation and is sometimes also known as “the beak star”.
The brighter component in Albireo is called Albireo A and is a binary star with two stars only 9.4 arc seconds apart. It is a yellow star with an apparent magnitude of 3.18 and the system belongs to the spectral class K3III .
The fainter component is Albireo B, a blue companion star with an apparent magnitude of 5.82 that belongs to the spectral type B0V. It is a fast-rotating Be star, with an estimated rotational velocity of 250 kilometres per second.
Deep Sky Objects In The Northern Cross Asterism
North America Nebula
The North American Nebula, also known as NGC 7000 or Caldwell 20, is an emission nebula with an apparent magnitude of 4. It is located just to the northeast of Deneb. This nebula is very large, measuring about 120 by 100 arc minutes, but cannot be seen without binoculars because its surface brightness is pretty low. It is obscured by a band of dust, which determines its shape as we see it.
The North American Nebula is a part of the same H II region, which is a large gas cloud in which star forming activity takes place, as its neighbouring nebula the Pelican Nebula. It lies about 1,600 light years away from Earth and was named the North American Nebula as its shape resembles the continent of the same name.
Messier 29, also known as M29 or NGC 6913, is an open cluster that can be seen with binoculars. It lies north of Deneb. It has an apparent magnitude of 7.1, with the brightest star having a visual magnitude of 8.59. The five hottest stars in the cluster belong to the spectral class B0.
Messier 29 is thought to be around 10 million years old. It is around 4,000 light years away from us and was first discovered by Charles Messier in 1764.
Messier 39, also known as M39 or NGC 7092, is an open cluster that is located about 800 light years from the Solar System. It lies south of Sadr and can be seen with binoculars.
M39 has an apparent magnitude of 5.5, with the brightest star having a visual magnitude of 6.83 and belonging to the spectral class A0. The brightest stars will soon evolve to the red giant stage.
It was first discovered by Charles Messier in 1764 and is thought to be between 200 and 300 million years old.
The Veil Nebula is a supernova remnant around 90 light years in size. It is thought to be between 5,000 and 8,000 years old and is located approximately 1470 light years away from Earth, near Aljanah.
Cygnus X-1 lies about halfway along the Swan’s neck. It is an x-ray source, famous for being one of the strongest ones seen from Earth. It is also the first X-ray source widely believed to be a black hole candidate — it has a mass that 8.7 times that of the Sun, yet is too compact to be any kind of known object other than a black hole.
First discovered in 1964 during a rocket flight, Cygnus X-1 is roughly 6,100 light years distant from Earth. It orbits a blue supergiant variable star, HDE 226868, and the two stars form a binary system.
- Cygnus may be a significant part of the origin of the myth of the Stymphalian Birds, one of The Twelve Labours of Hercules, along with other avian constellations near the summer solstice.
- The Chinese associate Cygnus with a myth — the one of the “magpie bridge” — Que Qiao.
- Out of the 58 stars selected for navigation, Deneb is the only one selected from Cygnus.
- The constellation Cygnus is best viewed during the month of June.
- By Wesley Chang – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=97682867
- By Veryoldphotons – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=94410606
- By Jim Mazur – http://www.skyledge.net/Messier39.htm, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=96267497
- By Ken Crawford – http://www.imagingdeepsky.com/Nebulae/NGC6960/FullSizeJpg/NGC6960.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=30574320
- By ESA, Hubble – https://apod.nasa.gov/apod/ap080811.html, http://www.spacetelescope.org, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6707568