June 2011 Sky from the Keeble Observatory Last month, we were going to see how the Type Ia supernovae provide us a “standard candle” for measuring the size of the Universe. However, the realities of end-of-semester and final projects and final exams meant that the intended May posting never happened. My apologies!Let’s recall some key points in the evolution of “low mass” stars like our Sun. These spend billions of years as main sequence stars, fueled by the fusion of hydrogen into helium at their cores. In the case of the Sun, after 10 billion years the hydrogen at the core is exhausted. Although hydrogen continues to fuse in a shell around the core, the helium core cannot fuse until the temperature rises substantially. This happens after the “shell source” has added enough additional helium. The star at this time is a red giant, swollen in size to engulf the orbits of its inner planets, if any. (We’ll be toast by then!) Helium fusion at the newly ignited core makes carbon (mostly) and oxygen, but this is the end of the line for energy production. The star will eventually eject its hydrogen envelope, leaving behind a remnant ball of degenerate carbon – a white dwarf – with almost the mass of the Sun but compressed into a sphere the size of the Earth.
In the 1920s, Subramanian Chandrasekhar showed that there is an upper mass limit for a white dwarf. Beyond 1.4 solar masses, the white dwarf will collapse and explode as a Type Ia supernova. So, where do we get this extra mass to turn 1 solar mass into 1.4? Recall that most stars are not single, but exist in binary or even triplet systems. Remember, also, that more massive stars evolve faster. Here, then, is the scenario:
The more massive star in a binary completes its main sequence and red giant phases, leaving behind a white dwarf. Its companion eventually swells into a red giant, and the white dwarf captures matter from the extended envelope of that star. If enough is transferred to exceed the Chandrasekhar limit … BOOM! The explosion will outshine an entire galaxy, even exceeding the luminosity of the massive star detonations known as Type II supernovae.
It is their brightness and the precision of the Chandrasekhar limit that makes Type Ia supernovae such excellent standard candles. Since the mechanism is the same, they all have the essentially the same peak brightness. Measure the apparent brightness and compare with that peak. The comparison tells you how far away the explosion was.
Lunar phases for June: New Moon on the 1st, at 5:03 pm; First Quarter on the 8th, at 10:11 pm; Full Moon on the 15th at 4:14 pm; Last Quarter on the 23rd, at 7:48 am.
Pre-dawn planet watchers can concentrate on the eastern horizon. Early in June you will see Jupiter rise about 90 minutes before sunrise, followed an hour later by Venus and Mars – though the angle of rise will be shallow, and you may not be able to see them in the horizon clutter and glare from the Sun. By the end of the month you’ll have an earlier shot at Jupiter, rising 3 hours earlier than the Sun. Mars will be an hour ahead of the Sun. Venus and Mercury will be too low to pick out of the glare.
Evening planet watchers will have to content themselves with Saturn, which emerges from twilight high to the south. It sets about six hours later. By the end of June, look for it a bit further west, and setting about five hours after sunset.
Looking straight overhead at mid-month, about two hours after sunset, you will find the constellation Bootes (the Herdsman). Its brightest star is Arcturus, about 20 degrees southwest of zenith. To the northwest, handle high and almost vertical, you’ll see the familiar shape of the “Big Dipper” in Ursa Major. To the west, look for the familiar “sickle” of Leo. Its brightest star, the heart of the Lion, is Regulus, about 20 degrees above the horizon. Virgo lies to the southwest. Spica is its brightest star, about 20 degrees left of Saturn. Low to the south you will see Scorpio, with the bright red Antares about 20 degrees off the horizon. East and northeast, look for the Summer Triangle of three bright and widely spaced stars – Deneb, Vega, and Altair. Cygnus, the Swan is to the northeast, with Deneb at about 30 degrees altitude. To the east, bright Vega marks Lyra, about 50 degrees from the horizon, and Altair in Aquila is below at about 20 degrees. The Summer Triangle will climb higher as we move through the long, hot summer.
Copyright 2011George Spagna