COMA BERENICES
COMA BERENICES

Students:
Maria Valentinova Krumova
Pavel Dimitrov Tashev
Veselin Tonchev Todorov
A group from Astronomical Observatory and Planetarium Nicholas Copernicus in Varna , Bulgaria.
Teacher: Eva Stefanova Bojurova

THE CONSTELLATION.
The constellation Coma Berenices, Berenice's Hair, is visible in the northern hemisphere in spring and summer and may be found between Virgo and Ursa Major. Its name refers to a classical story of love and honor.
Mythology.
According to mythology, Berenice was the daughter of royalty. She was married to her brother, Ptolemy Euergetes, an ancient King of Egypt. When he went off to war, she offered her hair as a sacrifice to gods, if they grant him a safe return. As Ptolemy returned successfully, his wife had her beautiful tresses ceremoniously clipped and placed them on the Altar in the temple of Venus. As the evening's festivities continued, the locks were stolen and placed among the stars. Some say Venus herself was the thief, while other think it was just the Greek astronomer Konon of Samos (247 BC), who made up the whole story in order to comfort Berenice when she heard of the theft. Some other versions of the story said that the hair was turned into a hair-star, or comet. The origins of the constellation's actual position in ancient times are actually unknown, some controversy surrounded the location, until the great Danish astronomer Tyho Brahe (1546 - 1601) in 1602 settled the matter by recording its present position. The ancients had the constellation overlapping with either Leo's tail or Virgo.

Although lacking quite bright stars, the constellation is unusually beautiful because it contains the remarkable Coma Star Cluster. There are also several interesting double stars, variable stars, the globular star cluster M53 and the famous cluster of galaxies. The galactic northpole is located in this constellation.

How to find Coma Berenices on the sky ?
Coma Berenices is surrounded by the constellations of Canes Venatici (the Hunting Dogs) to the north, Virgo (the Virgin) to the south, Leo (the Lion) on the west border and Bootes on the east border.
Brightest stars. From Denebola (Beta Leonis) draw a line to the bright star to the southeast, Arcturus (Alpha Bootis). Alpha Comae, the second brightest star in the constellation is found on this line at about the midpoint. It is sometimes called Diadem. It has the same diameter as our Sun, and is 62 light years away with a luminosity of 4.2 and is a class F5 star. It is an eclipsing binary with the companion having a magnitude of 5.
Now proceed north from Alpha Comae to Beta Comae and then west about the same distance to Gamma Comae. These three stars form half of a nearly perfect square. They aren't very prominent, and you will have to have a nice dark night in order to study them.
Beta Comae is actually the brightest star in the constellation, and certainly the closest at 27 light years. It too has a diameter equal to the Sun.
Gamma Comae is an orange star about 260 light years away. It is in the same region as the well-known Coma Star Cluster, but isn't member of that group.
The cluster of galaxies. The Coma Berenices cluster of galaxies is at a distance of about 90 Mpc and has an angular diameter of about 4 degrees on the celestial sphere as observed from Earth (corresponding to a physical diameter of ~ 6 Mpc). It contains approximately 10,000 galaxies, most of which are faint dwarf ellipticals.
M100 M109
M95
NGC3628
NGC4314

THE COMA STAR CLUSTER
General information. This large and conspicuous open star cluster was first cataloged by Ptolemy about 138 AD. It is scattered over an area of about 4.5 degrees diameter. Although conspicuous, its nature as a true, physical cluster was proven only in 1938 by R.J.Trumpler who identified 37 stars as true cluster members. Prior to this, P.J. Melotte had cataloged it in his 1915 catalog as No. 111.
Best seen in binoculars, the cluster fills the entire field of view: about 40 stars spread out over a five degree area. The cluster was once known as the tuft of hair at the end of Leo's tail. It now constitutes Berenice's golden tresses.
The cluster is one of the closest to our solar system. Its distance has recently been refined by data of ESA's astrometric satellite Hipparcos, and is now estimated at 288 light years. The brightest member of the cluster is 12 Comae. Other fourth-magnitude members are 13 and 14 Comae, and another thirty or so fainter stars go to make this one of the loveliest sight in the heavens.

What is an open star cluster? It is a gravitationally associated collection of stars, numbering between twenty and one thousand components. They have a common origin being born together in a cloud of interstellar gas and dust. The stars belonging to a cluster have a common proper motion (a shift that is discovered by photographing one and the same sky area in different moments of time) relative to other stars. Astronomers can measure this motion and distingush between the stars that are members of the cluster and the other stars. The photographic process can also reveal the physical or angular size of the cluster and the approximate number of stars.

PRACTICAL EXERCISE - COLOUR MAGNITUDE DIAGRAM
Most important of all for the astronomers is the production of the Colour-Magnitude Diagram. From this diagram, first realised in the 1920's, the ages of the stars within the cluster can be calculated, including the age of the cluster itself. Furthermore, we can extend this data to include deduction of stellar masses, sizes, luminosities and densities.
      The luminosity of a star is the full energy it emits in all directions per unit of time. It depends on the star's radius and temperature. The hotter the star the more luminous it is. And apparently the stars with larger radii would have larger emitting surfaces and higher luminosities too.
      The stellar magnitude measures the apparent brightness of the star which depends on its luminosity and its distance to us. The lower the magnitude, the brighter the star we see.
      The temperature of a star is determined from its spectrum. The spectrum is obtained using a telescope equipped by a spectrograph - an instrument dispersing the stellar light into different colours corresponding to different wavelengths.
UVES is a two-arm crossdispersed echelle spectrograph covering the wavelength range 300 - 500 nm (blue) and 420 - 1100nm (red), with the possibility to use dichroics. The spectral resolution for a 1 arcsec slit is about 40,000. The maximumresolution that can be attained with still adequate sampling, using a narrow slit, is about 110,000 in the red and 80,000 in theblue.
      The colour of a star depends on its temperature. The hottest stars emit most of their energy in short wavelengths and that is why they look blue. Then come the white, yellow, orange stars. Red stars are coolest. They emit mostly in long wavelengths. The colours of the stars can be "measured". Astronomrs do it by comparing the stellar magnitude of a star observed through a blue filter and denoted by B, with the magnitude obtained through a yellow-green filter and denoted by V. The value B - V is called a colour index. The stars with lower B - V values emit more intensely in short wavelines, and hence they are "bluer", i.e. hotter stars. The stars with a high colour index are cooler. ("redder").
      The Hertzsprung-Russell Diagram , pioneered independently by Elnar Hertzsprung and Henry Norris Russell, plots Luminosity as a function of Temperature for stars. Below is the Hertzsprung-Russell (HR) Diagram for stars near the sun:



The majority of stars fall along a curving diagonal line called the main sequence but there are other regions where many stars also fall. The H-R Diagram is an extremely useful way to follow the changes that take place as a star evolves. Most stars are on the Main Sequence because that is where stars spend most of their lives, burning hydrogen to helium through nuclear reactions. Stars are arranged along the main sequence by their masses. At the lower right end of it are the lightest stars, upper left are the heaviest. As stars live out their lives, changes in the structure of the star are reflected in changes in stars temperatures, sizes and luminosities, which cause them to move in tracks on the H-R diagram.
      Colour-magnitude diagram of the Coma cluster. In the table bellow you see data about the stellar magnitudes,which are changes, to be like if there are about 59.7kly from us(like M53) and the colour indices of a number of stars in the Coma cluster.
V B-V
19.66 0.4
21.45 0.65
22.74 0.47
19.06 0.14
23.38 0.55
19.43 0.35
19.16 0.45
23.28 0.94
18.06 0.18
21.92 0.41
22.93 0.44
21.63 1.14
19.37 0.3
20.38 0.5
V B-V
23.26 0.52
21.96 0.62
20.63 0.57
19.64 1.06
21.85 0.53
22.24 0.36
21.13 0.44
23.08 0.57
23.09 0.46
18.87 0.88
19.02 0.28
20.48 0.46
17.78 0
23.17 0.44
V B-V
21.93 0.9
22.45 0.59
22.05 0.71
20.15 0.51
20.68 0.57
20.69 0.11
22.53 0.53
21.81 0.56
20.22 0.95
20.88 0.59
20.1 0.47
19.44 1.34
20.03 1.42
23.05 0.54
V B-V
22.84 0.57
21.25 1.09
19.94 0.44
21.33 0.8
16.53 0.27
19.83 1.03
22.66 0.44
22.88 0.46
18.29 0.19
21.68 0.8
22.19 1.02
18.23 0.22
16.87 -0.06
18.12 0.16
V B-V
22.61 0.51
19.95 0.45
21.95 1.19
20.69 0.48
20.92 0.62
16.76 0.08
18.27 0.22
18 0.27
22.79 0.51
18.99 1.06
21.86 0.29
19.71 0.52
19.82 1.09
20.88 0.46

Using these data we can create a colour-magnitude diagram of the Coma cluster. It is the same as the H-R diagram, because the values B-V are a measure for the stars temperatures, and the apparent magnitudes are a relative measure for their luminosities. The latter is true since the stars belonging to the cluster are at approximately equal distances from us.

Our graph looks quite different from the H-R diagram shown above. In fact only the upper left part of the main sequence is seen. And lower, to the right of it a number of stars have a strange location. It showes that this star cluster is very young. The Sky Catalog 2000 gives an age of only 400 million years (the age of our Sun is about 5 billion years). Only the most massive and quickly evolving stars were able to form and take their places in the upper left end of the main sequence. The lower part of it is missing. Instead, there are stars to the right of the sequence who are in fact not "true" stars yet. They are still protostars in the phase of their formation. In the future they will evolve to take their places in the main sequence but it will happen when their cores become hot enough for the nuclear reactions to start and create a stable state.

COMPARISON WITH A GLOBULAR STELLAR CLUSTER
In the same constellation of Coma Berenices we have a more distant stellar cluster of another type - the globular stellar cluster M53. It is much denser and more populated than the open Coma cluster.The distance to the globular stellar cluster is about 59,7 kly..


Here is its colour-magnitude diagram:


It is very different from that of the Coma cluster. The upper left end of the main sequence is missing, and only the lower right part is there. There are a lot of red giant stars to the right. It is because this cluster is very old. The most massive stars (who usually stay in the upper right part of the main sequence) have already gone through the stable stage. Later when they exhausted their hydrogene fuel, they became red giants, and now only super dense black holes, neutron stars and white dwarfs remained. These objects were too faint to be observed and are not shown on the diagram.The lightest stars which evolve very slowly are still on the lower right part of the main sequence. Going to the upper left the sequence ends with the stars who leave from it for the red giant area just now. They form the so-called turn off point of the main sequence which allows us to estimate the age of the cluster. The globular star clusters contain the oldest stars in our Galaxy. Their ages are up to 15 billion years.

We are very thankful to this project, because it put us on our mettle to learn many things about stars, stellar clusters, their formed and their age.Now we know much more things about astronomy at all and love it twice! We hope that our project will be so interesting and useful to another people, too.

References:

  • Astronomical Calendar - Invariable part, Moscow, 1981, p. 694 (in Russian)
  • www.astro.wisc.edu
  • www.nasa.gov
  • www.dibonsmith.com
  • www.astronomical.org
  • www.seasky.org
  • www.csep10.phys.utk.edu
  • www.starryskies.com


You can contact us on this e-mails:
  • planetarium@triada.bg
  • maria_scorpio@yahoo.com