Regular readers of my Blog will know that I come more from the data analysis side of astronomy than the pretty pictures side. So this month just for a change, we’ll have some wonderful images as well as data. So, let’s start off with … a pretty picture!
In fact, Figure 1 is far from being ‘just a pretty picture’ – this stunning image shows two galaxies, NGC2788 and NGC2789 which are in collision.
How do we know these galaxies are interacting and that what we are seeing is not just a trick of perspective? If we look at the NASA Extragalactic database, we see the data in Table 1:
The first thing we can see is that both galaxies should be visible in modest sized amateur telescopes. The two galaxies have similar magnitudes, as shown above, although remember that each order of magnitude decreases brightness by approximately 2.5.
The fact that NGC2798 and NGC2799 are close together is revealed by two parameters: their redshifts are similar within two parts in 10,000. And their Hubble distances with respect to the Cosmic Microwave Background (CMB) are approximately only 2.78% different.
Will stars collide?
The NASA/ESA press release that accompanied this picture states “While one might think the merger of two galaxies would be catastrophic for the stellar systems within, the sheer amount of space between stars means that stellar collisions are unlikely”. How can that be established?
As a first approximation, we can model a spiral galaxy such as our Milky Way (MW) as a disk where the radius of the disk, is about 50,000 light-years or 15.33 kpc and the thickness of the disk is about 0.5kpc. The number of stars in the MW is estimated to be N ~ 10¹¹
We are now interested in parameter called the number density of stars - simply, the number of stars divided by the volume of the disk.
However, we can’t work out the volume of the disk simply by calculating the volume of an equivalent cylinder. In reality, the disk of a spiral galaxy is not homogenous – it has been known since the’60s that the spiral arms are density waves (Lin & Shu, 1964). Let's assume 70% of stars are in the spiral arms; there are no stars in the voids between the arms; and that the arms make up 50% of disk. These are of course gross assumptions, good only as a first approximation. The results (calculations are available in Excel® if you are interested) are shown in Table 2 below:
In the ‘Single galaxy’ column the number density of stars is less than one star per cubic parsec; expressed differently: in one cubic parsec of space, there is on average less than one star. In fact, on average we’d need to search 2.63 cubic parsecs of space to find a single star.
In the ‘Two similar colliding galaxies’ column, we imagine two spiral galaxies colliding head on. Here, the number density is doubled as the spiral arms collide. Correspondingly, the volume of space on average we expect to encounter a star is still 1.31 one cubic parsecs.
Let’s just remind ourselves how large a volume of space a cubic parsec is. Imagine a cube of space one parsec on each side. That’s 3.26 light-years on each side or, if you prefer, 3.1*1013 km on each side.
Even a very large star such as the red supergiant Betelgeuse is a very small object in all that space, so even in a galactic collision, as the NASA press release says, the chances of two stars colliding is very small.
How common are galaxy collisions?
The answer is – not uncommon. The next two images show some well-known examples.
The image in Figure 3 shows the distorted disk of NGC4631. This is caused by NGC4631’s interaction with two much smaller galaxies, NGC 4627 and NGC 4656.
The plot in Figure 4 shows the results of radio emissions of NGC4361 reported in Neininger & Dumke (1999). The small dark object above the disk of NGC4631 is the dwarf elliptical galaxy NGC 4627 with which NGC4631 is interacting.
Another source of data about colliding galaxies is Galaxy Zoo, a project that enables citizen scientists to categorise and analyse images of galaxies taken by professional observatories. One of Galaxy Zoo’s initiatives is the Galaxy Merger project (Holinchech 2016). The data collection phase is now complete and comprises 62 colliding galaxy pairs (I have this data as an Excel table for anyone interested).
A good example from the Galaxy Zoo merger table Is ARP148 in Ursa Major (Galaxy Zoo mergers table ID49). This is also called Mayall’s Object, named after American astronomer Nicholas U. Mayall (not John Mayall), who discovered the object in 1940.
Can it happen here?
There is a short answer: ‘Yes’. For example, the Sagittarius Dwarf galaxy has been involved in multiple collisions with our Milky Way galaxy and this is the probable reason for the warped nature of the Milky Way’s disk (Law & Majewski, 2010).
A recent determination of the radial velocity of the Andromeda galaxy, M31 with respect to the Milky Way indicates a velocity of −109.3 ± 4.4 kms per second, the negative sign indicating M31 is moving towards the Milky Way. The same research indicates a low transverse velocity of 17 km per second, indicating the probability of a head-on collision between the two galaxies (van der Marel, 2012)
The end result of this collision will be a very massive elliptical galaxy.
This Blog is not a scientific paper, although it lists various scientific papers as sources in the References section. I am indebted to Hugh Allen for drawing my attention to the paper by Neininger & Dumke (1999).
Data used in Table 1 was obtained from the NASA Extragalactic Database. The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Holincheck et al (2016).
Galaxy Zoo: Mergers – Dynamical models of interacting galaxies.
MNRAS459,720–745 (2016). doi:10.1093/mnras/stw649
Law, D; Majewski, S (2010). The Sagittarius dwarf galaxy: a model for evolution in a triaxial Milky Way halo.
2010 ApJ 714 1. DOI: 10.1088/0004-637X/714/1/229
Lin, C & Shu, F (1964).
On the Spiral Structure of Disk Galaxies. ApJ, vol. 140, p.646 . DOI: 10.1086/147955
Neininger, N; Dumke, M (1999). Intergalactic cold dust in the NGC 4631 group.
van der Marel, R et al (2012). The M31 Velocity Vector.II. Radial Orbit Towards the Milky Way and Implied Local Group Mass.
ApJ, 753:8 (14pp), 2012 July 1. DOI: 10.1088/0004-637X/753/1/8 https://iopscience.iop.org/article/10.1088/0004-637X/753/1/8/pdf
We haven’t been favoured with particularly good observational weather recently so let’s hope that with darker evenings following the change from BST to GMT we also get clearer skies.
The chart below represents the night sky at 10.00pm on the 8th November and at 9.00pm on the 23rd November. Best viewing of what is discussed will be towards the end of the month and going into December.
If you face south, as usual, and look directly overhead you will easily find the ‘W’ shape of the constellation Cassiopeia which we continue to enjoy on its journey westwards in the evening sky. Look to the west and you should see the bright star Deneb, the tail of the swan in the constellation Cygnus, as it flies to the western horizon. So, as Altair slips below the horizon and out of sight, it’s time to say goodbye to the Summer Triangle, but this month we are looking to the east because as one constellation sets in the western sky another one appears in the east.
This month we will have the arrival of the constellation Orion- The Hunter. It’s my favourite constellation because of its distinctive shape and because it appears to have everything. It doesn’t take much to visualise a hunter from the stars in Orion and what stars they are! Orion’s right shoulder is represented by the star Betelgeuse, a variable red supergiant, varying in magnitude from about 0.3 to 1.2 and the 7th brightest star in the northern hemisphere. If Betegeuse were to replace our sun it would reach out all the way to the orbit of Jupiter. It also has the potential of going supernova but of course we do not know exactly when. Then, representing his left foot, is the blue supergiant Rigel the 5th brightest star in the northern hemisphere with a magnitude of 0.2. Between these stars is a line of three stars going from south east to north west and they represent Orion’s belt and at magnitudes of around 2 they are unmistakable. Less bright but still visible to the unaided eye is Orion’s sword hanging from his belt. The bottom star of the sword should be visible in good conditions and above this is a misty fuzzy patch which is the Orion nebula (aka M42) where star formation takes place. Try to observe it through binoculars or a telescope if you get the chance.
Because it is so easily recognisable, Orion is a good starting point for finding your way about the night sky during the winter months. Follow a line from Orion’s belt to the upper right, underneath the star Bellatrix representing his left shoulder, and you will find the star Aldebaran, a giant red star of magnitude 1 and the 9th brightest star in the northern hemisphere. It is in the constellation Taurus- The Bull, and is said to represent the angry eye of the bull. The ‘V’ shape of stars outlining the bull’s face is an open star cluster called the Hyades. Continue the line beyond Aldebaran and you find the better known star cluster- The Pleiades or Seven Sisters. Remember back in springtime we watched Venus pass close to the Pleiades which catches the unaided eye but much more is revealed if you use a pair of binoculars.
Bright stars are like the proverbial bus, you wait ages to see one then four come along at once. Our fourth star this month is Capella in the constellation Auriga- The Charioteer, lying directly above Taurus. Having followed the line to the Pleiades turn ninety degrees to the north and the bright star you see is Capella. It is the 4th brightest star visible in the northern hemisphere and shines at magnitude 0.1. Auriga is in the shape of a pentagon although the most southerly star is in Taurus.
As mentioned earlier the constellations and stars described above are presently in the east in the evening and will be better viewed later on but are highlighted so that you can enjoy them throughout the winter months.
Something to look out for
Mars continues to be an attraction and it will have a close approach with the Moon on the 25th November and this will visible throughout the evening.