July 2011 Sky from the Keeble Observatory
Several months ago, we looked at the nuclear processes which fuel “main sequence”
stars and concluded that, even for massive stars, the production of iron was the
“end of the line” for releasing energy by fusing light elements into heavier ones:
hydrogen to helium, helium to carbon and oxygen, etc. A reader points out that iron
is fairly early in the periodic table – it’s number 26 out of more than 110 known
– and very low mass compared to the even common heavier elements like silver, gold,
mercury, or lead. So, he asks, where do the other elements come from?
Most are familiar with the periodic table of the elements, something you
were probably introduced to in middle school science classes. We see hydrogen and
helium, the two lightest and most abundant elements, in the upper left and right
corners. Arrayed below and between these two, we see 18 columns and 7 rows enumerating
the known elements, with two additional rows displayed that should be tucked into
the 6th and 7th rows. They are arranged by atomic number,
which counts the number of protons in the nucleus, and generally show increase in
atomic mass with atomic number. The mass is roughly the total number of protons
and neutrons in the nucleus. For low mass elements, the number of protons is approximately
the same as the number of neutrons – for example, the most common isotope of oxygen
has 8 of each. For heavier and heavier elements, however, the number of neutrons
grows more rapidly than the number of protons. For the most stable isotope of iron,
there are 26 protons and 30 neutrons. For the fissile isotope of uranium, there
are 92 protons and 143 neutrons.
Each element occurs in several isotopes – the same number of protons, but with different
numbers of neutrons. Isotopes with “too many neutrons” are subject to radioactive
transformation in what is called beta decay. One of the neutrons spontaneously changes
into a proton and ejects an electron and a weird little particle called a neutrino.
This changes the element to one of the next higher atomic number. If we bombard
a stable isotope with neutrons, we can turn it into an unstable isotope subject
to beta decay.
And this is where we make most of the periodic table. Some of the nuclear reactions
which take place in red giant stars release a steady but slow flux of neutrons.
Let’s start with iron-56, produced by fusion in the core of some massive red giant
star. It captures a neutron and becomes iron-57, which repeats the process and becomes
iron-58, which repeats again to become iron-59. This last isotope is beta-unstable
and decays to become cobalt-59. The process repeats with cobalt to make isotopes
of nickel, then copper, and on through the periodic table. The abundances produced
depend on the flux of neutrons, the rate of neutron capture, and the half-life against
This process of slow neutron capture (informatively known as the s-process!)
can produce the elements through lead-206, with the relative abundances of isotopes
in the proportions we find them in the solar system. If that weren’t confirmation
enough, we see in the spectra of some red giant stars the signature of an element
known as technetium – which has no stable isotopes and does not occur naturally
on Earth. It should be produced by the s-process, and the fact that we see it in
red giants requires it to be produces within the last 10,000 years.
To make elements heavier than lead requires a rapid flux of lots of neutrons … hence
the name r-process! Where would we get such a neutron flux? In the same supernova
explosion which destroys the massive star and scatters these elements back into
interstellar space to be incorporated in the next generation of stars. Our own Sun
was one such star, and all the elements other than hydrogen and helium were produced
in previous stars. As Carl Sagan was fond of saying, we’re all made of stardust!
Lunar phases for July: New Moon on the 1st, at 4:54 am; First
Quarter on the 8th, at 2:29 am; Full Moon on the 15th at 2:40
am; Last Quarter on the 23rd, at 1:02 am, and a second New Moon on the
30th, at 2:40 pm. While the second Full Moon in a month has come to be
called a Blue Moon, I’m unaware of any special name for the second New Moon.
Predawn planet watchers will notice a parade climbing from the eastern horizon.
Jupiter rises about 3 hours before sunrise early in the month (3.5 hours by month’s
end), and climbs to about 40 degrees above the eastern horizon at dawn. Mars is
lower and to the left, bright and unmistakably red. Venus rises about 30 minutes
before sunrise, but will be extremely bright and easy to see if your eastern horizon
is uncluttered. By month’s end, Venus will be lost in the Sun’s glare. Jupiter is
high to the southwest at dawn, having risen around midnight.
Emerging from evening twilight, Mercury will be bright about 15 degrees above the
western horizon early in the month, disappearing into the Sun’s glare by the end
of July. Saturn begins the month to the south-southwest, about 45 degrees above
the horizon at twilight, settling lower as the weeks of July pass.
Our overhead view at midmonth, about 3 hours after sunset, finds bright Vega near
zenith in the constellation Lyra. Recall last month’s note about the “summer triangle”
of Vega, Deneb, and Altair. Deneb is the bright star at the “tail” of Cygnus, the
Swan. Look for it about 60 degrees and to the east-northeast. Altair is at about
50 degrees to the southeast. Following the familiar t shape of Cygnus from Deneb,
your binoculars will find Albireo at the head of the swan. It’s not a terribly bright
star, but it lacks any bright neighbors so it’s easy to find – it lies about 70
degrees above the east-southeast at this time. A small telescope reveals Albireo
as a close binary, with the two stars of very different colors – one reddish, the
other bright blue. The redder star is cooler in temperature.
Ursa Major is to the northwest, with the bowl of the “Big Dipper” oriented to hold
water. This asterism is also known in some cultures as “the Plow” – the reason is
obvious in this orientation. Turning to the south we find the “teapot” asterism
of the constellation Sagittarius, which marks the direction toward the center of
our Milky Way galaxy. Above and a bit to the right of Sagittarius, don’t mistake
bright red Antares for Mars. The name itself means “against Mars” – though I doubt
there’s any real rivalry. Mars is a ball of rock in our solar system, the 4th
planet from the Sun. Antares is a “class M supergiant” star about 600 light years
from the Sun and about 800 times the Sun’s diameter. If the Sun were placed at the
center of this star, Antares’ photosphere (its visible surface) would lie between
the orbits of Mars and Jupiter.