A note on our May and June topic: BICEP2 researchers are now acknowledging that foreground dust in our own Galaxy may have biased their results, leading to an overconfident claim of discovering an observational signature for inflation in the early universe. Astronomers are still waiting for the (hopefully) definitive data set from the ESA’s Planck probe.
Do you remember the story of Goldilocks, and the recurrent theme whether beds or oatmeal that only one of three items was “just right?” This is also a recurrent theme in the search for exoplanets – i.e. planets orbiting stars other than our Sun. In particular, astronomers are interested in finding planets orbiting at the right distance for water to be possibly found in its liquid state, because liquid water is a prerequisite for the existence of life as we know it.
The luminosity of a star depends directly on its surface temperature (technically the photospheric temperature) and its radius, both of which are actually determined by the star’s mass and age. The brightness per unit area of the photosphere is proportional to the 4th power of its temperature, and the area is proportional to the square (2nd power) of its radius. This radiation spreads out in space, falling in intensity with the inverse square of distance, and it is this falling intensity which ultimately determines the temperature of a planetary body orbiting the star. Another important factor is whether the planet has an atmosphere – Earth is clearly habitable while the Moon is not. We call the range of distances where water can be liquid the habitable zone, or more colloquially the Goldilocks zone. Note that for a hot extremely luminous star this zone will be far from the star; for a cool dim star it will be close. But, whether near or far from a hotter or cooler star, the incident radiation on a planet in this Goldilocks zone will be about what we experience on Earth, around 1 kilowatt per square meter.
NASA’s Kepler mission has been used to identify over a hundred candidates for this distinction, though only about 25 have been confirmed to date. Analysis continues on the rest of the data set, so expect the numbers to rise. Most of the candidates are relatively close to small cool stars due to an observational bias – they’re easier to detect with the method used by Kepler.
One of my summer research students is working from the Kepler database to identify and categorize those stars which have potentially habitable planets. From their surface temperatures he is calculating the relative percentages of the red and blue light used by chlorophyll for photosynthesis in native Earth plants. He is also calculating the percentage of ultraviolet light. He will then attempt to simulate these proportions of red, blue and ultraviolet using filters and an ultraviolet lamp, and will attempt to germinate and grow from seed a variety of plants under “Earthlike” conditions, “cool star” conditions, and “hot star” conditions.
I’ll share his results at the end of the summer.
Lunar phases for July: First Quarter on the 5th, at 7:00 am EDT; Full Moon on the 12th, at 6:26 am; Last Quarter on the 18th, at 9:09 pm; New Moon on the 26th, at 5:43 pm.
The new moon late in the month means a good chance of seeing the delta Aquarid meteor shower, which peaks the 28th and 29th. Best time for viewing is after midnight as our horizon rotates into the stream of debris. Best place is away from town and city lights. Aquarius rises about 11 pm, and serves as the “radiant” for the shower, i.e. the direction from which they appear to emanate. Once your eyes are adapted to the dark, you may be able to see 20 or more meteors per hour streaking across the dome of the sky.
Predawn planet hunters may content themselves with seeing Venus to the east, about 20 degrees off the horizon at sunrise. Mercury will return to the predawn east by month’s end, but will be very low and likely lost in the Sun’s glare even if not lost in horizon clutter or haze.
Evening provides a continued opportunity to view Mars and Saturn, both of which start the month emerging from twilight towards the south. Saturn will be about 40 degrees off the horizon; Mars will be to the southwest and a little lower than Saturn. That bright star to the left of Mars early in July is Spica, the brightest star in Virgo. You can follow Mars’ eastward drift through the month as it passes close to Spica at mid-month, and finishes to the left by the end of July.
Our overhead view at midmonth, about 3 hours after sunset, finds bright Vega near zenith in the constellation Lyra. The “summer triangle” of Vega, Deneb, and Altair is easily seen to the east of zenith. 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.
If readers have questions about astronomy or science in general that you would like to see covered in one of these columns, please feel free to contact me at email@example.com.
Copyright 2014George Spagna