May 2002 Sky from Keeble Observatory

When confronted with the discovery of the muon, a subatomic particle that appeared (at the time) to have no purpose in the grand scheme of things, physicist I.I. Rabi is reported to have asked, "Who ordered that?" We might, perhaps, ask the same question about two apparently peculiar objects reported by observers using the Chandra X-Ray Observatory.

Recall that, in an earlier discussion, we described the possible end state of stars like the Sun as a compact, dense, object called a white dwarf. About the size of the Earth, but as massive as the Sun, a white dwarf represents the cooling core of a star with its nuclear fuel (hydrogen) exhausted. For stars a little less massive than the Sun, the white dwarf is essentially a ball of degenerate helium. For stars a little more massive than the Sun, the white dwarf is essentially a ball of degenerate carbon. For stars leaving a core more massive than 1.44 times the mass of the Sun, a white dwarf is not a possible end. Above this mass, calculated by Subramanian Chadrasekhar (affectionately known as Chandra, and for whom the Chandra Observatory is named), we expect an even more dense collapsed state, known as a neutron star. Think on the order of one solar mass, but of a size that would conveniently fit between Ashland and downtown Richmond. A bit more massive still, and we believe that even the neutron star is not possible, and the end state will collapse into a black hole.

Now the Chandra telescope has shown two objects which seem to represent an intermediate state between the neutron star and black hole. One, designated by its catalog number RXJ1866, is about 400 light years away, and seems to be too small to be a neutron star. The other, known as 3C58, is about 10,000 light years distant, and it appears to be too cool to be a neutron star. The first determination, for RXJ1866, hinges on knowing its distance (it's close enough for relatively simple geometry) and on the assumption that its surface is of uniform brightness. If there are "hot spots" we would have an incorrect measure of its luminosity, and would be underestimating its size. The evidence for 3C58 hinges on identifying this as the remnant of a supernova observed in 1181 AD - its temperature is not consistent with models of how fast a neutron star should cool, too low by a factor of 2.

What could they be? Speculation thus far has been that these represent a previously unseen form of "strange" matter. In the "Standard Model" neutrons are composed of quarks - in the jargon of high-energy physicists, a neutron is two down quarks and an up quark. Under high enough pressure, one of these quarks might be converted into a more massive strange quark. This strange matter would be more dense (hence would be a smaller object than a neutron star of the same mass) and would cool faster than "normal" neutron star material. Alternatively, we might have the wrong distance, we might not have a good handle on the cooling rate of dense nuclear matter, or the object we're seeing may not be the remnant of the supernova with which it's traditionally identified. Science is like that - and only more observational evidence will allow us to sort out the competing explanations.

Lunar phases for 2002 May: Last Quarter on the 4th, at 3:16 am EDT; New Moon on the 12th (Happy Mothers' Day!), at 6:45 am; First Quarter at 3:42 pm on the 19th; Full Moon on the 26th, at 7:51 am.

The visible planets remain evening objects. Morning observers will have to wait … there are no planets visible in the pre-dawn. Mercury is very low on the horizon after sunset, and will disappear into the solar glare by mid-month. As the twilight deepens, you'll be able to see Venus, Mars, Saturn, and Jupiter - on the 4th these latter four will be arrayed within about a 10 degrees toward the west.

Our overhead view at mid-month, about 45 minutes after sunset, finds the Big Dipper (Ursa Major) inverted high above the northern horizon. Turning toward the East, the bright star 45 degrees above the horizon is Arcturus. Low to the northeast but climbing higher into the evening sky is Vega. To the south we can find Leo, almost at zenith. To the west, dropping from our summer view, is Orion. Above that most familiar winter constellation is the bright pair Castor and Pollux. Look for these to form an isosceles triangle with Saturn at month's end. Sirius is low to the southwest, disappearing from view as Vega climbs into the northeast.