Well I’m pleased to say that the planets Mercury and Venus didn’t disappoint during the month of May. They were within one degree of each other on Friday 22nd although Mercury is challenging to spot unless you are located in a good site and your eyesight is quite sharp. They repeated with a more separated appearance on Sunday 24th but with the addition of a beautiful crescent Moon nearby. My eyesight isn’t what it used to be but I still managed to see Mercury naked eye. Seeing all three together was something special. This is a difficult time of year for astronomers as there is so little light free time and any local light pollution makes the matter worse.
We’ll start where we left off last month when we used the Plough to locate Polaris (the Pole star). You will notice that the Plough is not directly overhead anymore because Ursa Major is a circumpolar constellation and as it rotates about the Pole star, the Plough moves so that its pointer stars Merak and Dubhe keep pointing towards the Pole star and it changes its orientation in the sky so that looking north at present it appears to be standing on end. This is something to keep an eye on throughout the year until it returns to its original orientation in the sky.
Courtesy In-the-sky.org edited by B Davidson
So facing north, use the pointers, Merak and Dubhe, to find the Pole star and then from the third star in from the end of the Plough handle, Alioth, make a line through the Pole star and continue about the same distance beyond until you see a bright star. It will be the central star of a W formation, an asterism in the constellation Cassiopeia. Most people see the W shape and call it Cassiopeia. The bright star was never given a name in Western or Middle Eastern culture so is referred to as gamma (g) Cas. The convention is to name stars using the letters of the Greek alphabet and an abbreviated form of the constellation. Generally this is done in the order of brightness of the star but it is not a hard and fast rule.
However this star has been given the name Navi, allegedly by the American astronaut Virgil (Gus) Ivan Grissom as an anagram of his middle name because it was used for navigation in the early space missions. A fitting tribute to someone who made the ultimate sacrifice for space exploration. The constellation Cassiopeia is circumpolar and because it is directly opposite the Plough across the North Celestial Pole the two will have exchanged positions in six months so we will see Cassiopeia much better in November. As we will the other circumpolar constellation shown on the diagram, Cepheus, which is rather indistinct at present suffering from being too close to the horizon, the lack of proper darkness and the Bristol glow when looking north.
Now let’s go in the opposite direction. Follow the arc of the handle of the Plough round to the star, Arcturus which has the distinction of being the second brightest star visible in the northern hemisphere. Also known as alpha(a) Boo.
Courtesy In-the-sky.org edited by B Davidson
It is the brightest star in the constellation Bootes (The Herdsman) and again it is difficult to distinguish such a figure whereas the Kite asterism is easier to see. Carry on following the curve of the arc for about the same distance until you see a bright star on the Ecliptic. This is Spica the brightest star in the constellation Virgo (The Maiden). It may be easier to memorise this procedure using the expression "Arc on to Arcturus and Speed on to Spica". Now that you are on the ecliptic you can follow it round to the west (the same path followed by the Sun earlier in the day) and from last month you should recognise Regulus in the constellation Leo. So face west to Regulus then look up and you are back at the Plough.
Something to look out for
It is a challenging time for observing the skies when there is so little darkness but it is the summer solstice on June 20th so things will start to improve from then onwards. What about some daytime observation. Our favourite planet at present, Venus, is approaching inferior conjunction, the point in its orbit when it lies between the Earth and the Sun so we cannot see it during the first half of June but it soon makes an appearance in the morning sky and on June 19th it will be close to the waning crescent Moon at dawn. That would mean an early rise! It will be occulted by the crescent Moon (ie the Moon will pass between us and Venus) from 8.35am (BST) onwards but unfortunately it will not be visible to the naked eye. Perhaps some of our imaging friends will try to capture the event but great care needs to be taken as the Sun is up and in the same direction. It is a C shaped waning crescent so Venus will disappear behind the crescent then reappear about an hour later.
So what is Dark Matter?
Presumably there must be some kind of exotic particles that constitute Dark Matter (DM). The ‘Standard Model’ of particle physics looks like this:
It turns out there are candidates for DM in this model. ‘Neutrinos (the ‘e’, ‘μ’ and ‘τ’ in the leptons group) are DM candidates. Millions of neutrinos pass through the Earth and through our bodies every second, only very rarely interacting with matter. However, neutrinos have an extremely small mass and there are not nearly enough of them to account for the amount of DM required. One group of researchers postulates ‘sterile neutrinos’ which supposedly only interact with other neutrinos, arising when an ordinary neutrino morphs into a sterile neutrino. These results are highly contentious in the community, so neutrinos may only offer a partial explanation.
Another theoretical possibility is called a ‘Massive Compact Halo Objects’ (MACHO), a body composed of normal matter whilst emitting little or no radiation. Possible MACHOs include black holes, neutron stars, red dwarf stars and brown dwarf stars, or even planets not associated with any stars. These would be very faint and emit mainly at infra-red wavelengths rather than optical. Some, not completely conclusive observational evidence for MACHOs has been obtained via gravitational micro-lensing observations. Future observations by the upcoming James Webb Space Telescope, which will observe in the infra-red, may detect MACHOs, but there is still a problem. Theoretical studies indicate MACHOs cannot comprise more that 20% of the required dark matter. Add to that the 3% of normal matter we can see, and we still have the question “where is the other 77%?”
Another DM candidate is a theoretical, non-baryonic particle named ‘Weakly Interacting Massive particle (WIMP). The characteristics of a WIMP are framed such that if they exist it would answer the question as to what DM is. The theory is that WIMPs ought to interact very weakly with baryonic matter. The inferred distribution of dark matter in our galaxy (i.e. the DM halo) shows a considerable contribution in our location, so as we move through space, we ought to pass through much DM. If DM is made of WIMPs, then we could directly detect the rare interactions between WIMPs and ordinary matter. The existence of WIMPs is allowed under an extension of the standard model of elementary particles called supersymmetry. The first problem with WIMPs is that supersymmetry theory has no observational basis. And the second snag; nobody has detected a WIMP.
The last current theory for DM postulates particles named Axions. As with WIMPs, the properties of Axions are framed such that they would account for DM. Because of these properties, axions would interact only minimally with ordinary matter. Axions are predicted to be electrically neutral, have very small mass and very low interaction cross-sections for the strong and weak nuclear forces. This would require modifications to Maxwell’s Equations. Axions would also change to and from photons in magnetic fields. Quite a wish list!
Current physics assumes gravity has always acted as it does now; acts the same everywhere; and under all conditions. Suppose that isn’t the case? The leading –though by no means widely accepted - alternative theory to DM is Modified Newtonian Dynamics (MOND) which postulates that under conditions of low acceleration, gravity behaves differently. It also asserts that the inverse square law, while being true over comparatively small ranges such as the solar system, is not applicable over galactic scales. While MOND appears to account for the motions of galaxies without the need for DM, it does not account well for the observed motions within galaxy clusters – reminding us of Fritz Zwicky’s 1933 DM conclusions.
MOND also flies right in the face of Einsteins General Relativity, which has passed every experimental test that has been thrown at it since 1917.
Most physicists believe DM exists. We do know what DM does. We have little idea about what DM is. Current explanations involve serious modifications of the Standard Model of particle physics, or serious modifications to General Relativity, maybe even both. It’s uncomfortable to think that we don’t know what most of the matter in the universe is. It’s an interesting time to be involved in astrophysics.
The author wishes to acknowledge the assistance of Bob Merritt in the preparation of this article.