M51 Whirlpool galaxy

Charles Messier discovered the M51 Whirlpool galaxy in 1773. His description of it as a “faint nebula without stars” does not exactly match the information that has since been gathered of the galaxy. When the sky is dark M51 can easily be spotted even by an amateur.

The galaxy's distance from ours is usually estimated to be between 30 to 40 million light years, but some estimates are as low as 20 million light years. The distance can be measured for example by using photometric methods. The distance of an object of a known size can also be measured by taking a photo of the object with a camera and using the focal length of the lens for calculation, which we have done. The diameter of M51 galaxy is calculated to be about 100 000 light years, but this depends on the estimated distance between the Milky Way and M51. The galaxy's mass is estimated to be about 160 billion suns. M51 is shaped like a spiral, which might be due to the galaxy's interaction with its neighbour, NGC 5195, which was discovered in 1781 by Pierre Méchain, who in fact was Messier's assistant. Both galaxies are located in the constellation Canes Venatici.

M51 received extra publicity in 1994, when amateur astronomers Jerry Armstong and Jim Puckett discovered a supernova in it. A supernova is a radical explosion in which a star is destroyed. According to the NASA press release, when the supernova was at its brightest it was 100 million times brighter than the sun. The supernova was intensely examined with NASA's Hubble Space Telescope and found to be of a very rare type (1c). As can be seen from pictures taken with large telescopes, there are bright spots in the spiral arms of the galaxy. In these bright spots massive and bright stars are forming. The nucleus of the galaxy has also been examined with the Hubble Space Telescope.

The larger image shows the location of M 51 in Canes Venatici
The smaller black and white image is an enlargement of a part of the bigger image, photographed with a microscope. The side of the square is about 1mm.

Photographs by Sakari Ekko

We used these images to measure the distance of M51 from the Milky Way. The focal length of the lens used is 200 mm and by measuring we found that the diameter of M51 on the original image is 0,5mm. Assuming that the diameter of M51 is 100 000 light years, the same as the diameter of the Milky Way, we can calculate the distance between M51 and the Milky Way. Two similar triangles are formed, so the ratio between the length and the width is the same for both triangles. From this we get : x= 100 000 ly * 200 mm/ 0,5 mm. From this, x= 40 million light years. This result is fairly rough due to the method used. From the original image we cannot see the outer parts of the galaxy, as they are quite dim. The diameter of M51 is also only an estimate, so the mistake in our result may be as big as 50%. However, our result is close to other estimated distances between M51 and the Milky Way, which have been measured using various different methods.

Comparison between M51 and the Milky Way

The main types of galaxies are spiral galaxies and elliptic galaxies. M51 and the Milky Way are both spiral galaxies. All spiral galaxies consist of both stars and matter between the stars. There are for example dust clouds and star forming regions. Seen face-on, M51 is a galaxy very similar to our own. They both have a diameter of approximately 100,000 light years. The masses of M51 and Milky way are equivalent to 160 and 200 billion suns, respectively, so their masses are about the same too.

The spiral structure of M51 was discovered in 1845 by Lord Rosse with his 72'' telescope, which at the time was the largest telescope in the world. This was the first time that a galaxy was found to have a spiral structure. The structure is thought to be a result of M51's encounter with its neighbour, and the nickname Whirlpool Galaxy describes its appearance well. The form of the Milky Way has been quite difficult to disentangle due to the dust between the stars which obscures the structure of the galaxy. The form of has been studied by radio observations. The stars of the Milky Way form a very flat disc and it has a spiral structure like M51.

M51 is the dominating member of a small group of galaxies whereas the Milky Way together with Andromeda and M33 galaxy are dominant in a group consisting of about ten galaxies. M51 has a companion, NGC 5195. They both have bright centres and are expected to merge. In general, interaction between galaxies is not unusual.

Practical exercise

a) Given the distance between the sun and the earth, the radius of the sun and the earth and the gravitational constant, calculate the mass of the sun, assuming that the earth's orbit is circular.

distance: 149,6 * 109 m
radius (sun): 6,960*108 m
radius (earth) 6380*103 m
gravitational constant: 6,67*10-11 Nm2/kg2
mass (earth): 5,974*1024 kg

b) The distance between the Milky Way and the m51 galaxy is 37 million light years. What is this distance in metres?

c) Given the estimated mass of the m51 galaxy and the Milky Way galaxy, calculate the gravitational force between these two galaxies.

mass(m51): 160* 109 suns
mass(Milky Way): 2* 1011 suns

d) Using your result from part c, calculate what the acceleration of m51 would be if the only force exerted on it was the gravitational force between the Milky Way and m51.



Siiri Anttonen, Karoliina Hartiala and teacher Sakari Ekko, Puolalanmäen lukio Turku, Finland