Messier 99 & 100
by Wayne Chillingworth

Background and Brief Introduction (by Nigel Foster)

In March 2005 Knowle AS became the first astronomical society in the UK to stage an observing session over the internet using the new two metre remote controlled Faulkes Telescope in Hawaii. The event was hosted by Warwick University and was a joint collaboration between the Society, the University and the Faulkes project (a privately funded initiative with the primary aim of enabling UK schoolchildren to participate in serious astronomy projects using world-class telescopes). The 90 minute session was led by Richard Beare (a member of all three organisations) and more than a dozen galaxies and other deep sky objects were successfully observed and imaged as well as the planets Jupiter and Saturn.

In this article one of our members, Wayne Chillingworth explains his reasons for selecting the galaxies Messier 99 and 100 for observation. In the event only Messier 99 made the final list for observation but nevertheless members and visitors alike will be very interested to read about these two fascinating members of the Virgo cluster. The source of much of the data quoted in Wayne’s article is Bunham’s Celestial Handbook.


M99 (NGC 4254)

A spiral Galaxy in Coma Berenices of type Sc (small nucleus, loosely wound arms). Discovered by Pierre Méchain on 15 March 1781 (Messier on 13 April 1781). 55 million l.y. away.

The main reason for choosing this object is that we know from other photographs made by large telescopes that it is an exquisite face on spiral galaxy However, it is very difficult to make out any of the spiral structure in small aperture instruments, even under good skies. This is because it is face on and is therefore relatively transparent, compared to more inclined galaxies, and even edge on galaxies such as NGC 891 in Andromeda, which some members of the society were able to observe last year from the dark site. This is similar to the effect that we encounter when observing planetary nebulæ, when we can see more of the outer ring of the gas shell 


due to the concentration of matter in our line of sight. So the only real chance we have of observing the spiral structure in any detail, other than looking at someone else’s images, is to have access to a large aperture telescope, a CCD camera, or both, which we have the privilege of having today.

M99 was the second nebula, or galaxy, to be recognised as being a spiral by Lord Rosse, who pioneered the discovery of spiral structures in galaxies. He observed it in 1848 as a three-branched spiral through his Leviathan telescope in Ireland, which was, and still is since its recent restoration, a 72 inch reflector, which equates to 183cm, only 17cm smaller in diameter than what we are using today. Lord Rosse, however, did not have the benefit of photographic plates, let alone a CCD camera, which illustrates how fortunate we are to have access to the technology we are using today.

It is the asymmetrical three-branched spiral form of M99 which interests me most, and I think this would be a good target to analyse with the luminosity software to see if we can detect any fragmentation within the nucleus, which might suggest an earlier collision with another member of the Virgo Cluster, of which M99 belongs, which might account for the third spiral arm.

It would not be surprising to find evidence to suggest that M99 has encountered a collision, because with a radial velocity of 2,354 km/sec, M99 has one of the largest red shifts of any member of the Virgo Cluster (the largest for any Messier object). This would give it a proper velocity of at least 1200 km/sec in a direction away from us. An inter-galactic boy racer if ever there was one.

However, M99 is not alone. There is another nearby galaxy in the cluster, M98 (NGC 4192), - which I don’t think is on the list to observe today but is more easily seen in small instruments due to it being edge on to us - which has a radial velocity of 195 km/sec (a blue shift), and therefore has roughly the same proper velocity as M99, but this one is moving towards us. So it is also possible to consider that these two might have crossed swords themselves, and therefore to imagine, somewhat romantically, that M99’s third spiral arm is the cosmic equivalent of a shattering lance belonging to one of two giant inter-galactic jousting knights.

M100 (NGG 4321)

A spiral Galaxy in Coma Berenices of type Sc (small nucleus, loosely wound arms). Discovered by Pierre Méchain on 15 March 1781 (Messier on 13 April 1781). 55 million l.y. away.

Like M99, the main reason for choosing this object is that visually through small aperture instruments it is a relatively disappointing object compared to photographical observations, especially through large aperture telescopes.

Unlike M99, M100 is rotating in a counter-clockwise direction. I have never understood why this is, so I would be interested to know if any of the experts here today can explain why it is that some galaxies rotate in one direction while others rotate in another.

Deep photographs of M100 reveal that it is much larger than it appears in conventional photographs, with a significant part of its mass lying in the faint outer regions, too faint to be detected by conventional images.

The Hubble Space Telescope has imaged M100 extensively, and has discovered many Cepheid variable stars. Cephids (named after the proto-type star Delta Cephei) are yellow supergiants in a fairly advanced stage of evolution, so that they have used up their hydrogen and helium and have become unstable, swelling and shrinking over a period of about 3 to 50 days.

There is a known correlation between their period and their real luminosity, and by comparing their real luminosity to their apparent luminosity we can accurately estimate their distance.

Take, for example, Delta Cephei itself: if we first obtain its period, which happens to be 5.366 days, and refer to the known relationship between period and luminosity, we know that its actual brightness is -2.9. We then take the median apparent magnitude of 4.65 to obtain a difference of 7.55 magnitudes. This in turn can be used to look up on a table the corresponding difference in light intensity, in this case 1,000 times. That is to say the star is about 1,000 times fainter than it would be if it was at the standard distance of 10 parsecs, or 32.6 light years away. The distance is therefore 32.6 multiplied by the square root of 1,000, which gives us a distance of 1,031 light years.

In this way, the distance of M100 has been calculated to be 56 (+/- 6) million light years away.

General Comments

Finally, I think it is worth mentioning here, that in relation to objects such as M99 and M100, I feel that large telescopes like these and their associated technology represent something of a double-edged sword to the amateur visual observer. There can be no doubt that they provide us with wonderful images of objects that otherwise we would never see, and it is a real privilege to have this opportunity today, but the availability of these images compared to that of drawings made at the eyepiece can spoil us somewhat, and this can often lead to unrealistic expectations and disappointment in people trying to observe these objects visually for the first time.

If any of you have ever shown someone else Saturn or The Great Nebula in Orion for the first time, you will know that their ‘wow’ is in stark contrast to their ‘oh’ when you show them a face on spiral galaxy, and it is only after the second or third visit to the eyepiece when they unconvincingly acknowledge that they ‘think they can see something’.

Nevertheless, as amateurs with limited budgets we make use of what we have available to us, and attempt to train our eyes accordingly, and I’m sure that you are all aware that the Society holds observing sessions at a dark site as often as conditions coincide with our abilities to abandon other personal responsibilities, which in reality equates to about 10 evenings a year, and everyone is welcome to join us, regardless of whether or not you own a telescope. If you think that you would be interested in coming along to endure the freezing conditions to observe some of these objects in a far less spectacular way than we are doing here today, please have a word with Nigel, Stephen or myself, and we can keep you informed of when we will be next likely to meet.