Starry Skies Above Santa Monica
June 14-20, 2000

Mirek Plavec

Emeritus Professor of Astronomy, UCLA

There Goes the Sun
The Sun will reach its highest point above the equator on Tuesday, June 20 – we will have the summer solstice, and after that, the Sun will slowly begin its return trip to the south. That Tuesday will be the longest day of the year, 15 hours and 25 minutes of sunshine at our geographic latitude. Against the background of the stars, the Sun will be crossing from Taurus to Gemini.
The Moon will be Full on the evening of June 16, which is very close to the date of the summer solstice: so while the Sun will culminate as high as possible for us at noon of June 20, the Full Moon will be very low in the sky at its culmination, around midnight: it will shine above the "Teapot" stars of Sagittarius, which would be fairly prominent if the Moon were not there…
Where Are the Planets?
Still projected fairly close to the Sun, especially Mercury, Venus, and Mars. Venus passed behind the disk of the Sun on Sunday, June 11; at that time it was also farthest from the Earth (being on the opposite side of the Sun than the Earth); this largest distance is 260 million km. After June 11, Venus formally becomes an evening star. However, the planet is so far from us that its angular deviation from the Sun will be increasing only very slowly, and we will have to wait several weeks before we can spot Venus as an evening star after sunset.
Jupiter and Saturn have escaped faster from the Sun – actually, the Sun has moved farther away from them, as we see it in the sky. Thus, the two big planets are emerging from the glare surrounding the Sun, and are becoming visible again – if you are ready to get up probably a bit earlier than usual. Take the morning of Saturday, June 17: the Sun rises by 5:41, and the two planets rise almost 2 hours earlier, around 3:50. They are very close together, under the star cluster of the Pleiades, and the brighter Jupiter is just a bit lower than Saturn.

Bright Stars in the Evening
With the Moon close to its Full phase, not much can be seen of the starry sky, especially from the city. As the sky is getting darker, three first-magnitude stars are disappearing at the western horizon: Capella, Pollux (with Castor which is only a bit fainter), and Procyon.
Around 9 p.m., on any clear evening, you will hopefully be able to see the following seven stars of the first magnitude: Regulus, Arcturus, Spica, Antares, Vega, Deneb, and Altair. The handle of the Big Dipper points towards Arcturus, and down to Spica. Regulus in Leo is dominating the sky above the west. You can spot the reddish Antares in the south-east (the Moon will be above it on the evenings of June 14 and 15). The Moon is of no help with the remaining three stars, passing too far south below them. Yet the bright bluish Vega should be easy to find high in the north-east. Deneb, a bit fainter, is farther to the horizon behind Vega, while Altair shines above the eastern point of the horizon.
What makes these seven stars prominent? There is no simple answer, since three factors determine the apparent brightness of a star: (1) its proximity to us in space, (2) its radius, which determines how big a ball the star is, and (3) its surface temperature, which strongly affects the amount of radiation emitted by unit area of the star’s surface.
In each of these three categories, there are different winners (and losers). Nearest to us in space (among these 7 stars) is Altair, 17 light years, then Vega, 25 light years, Arcturus, 37 light years, then Regulus, 78 light years. For the three distant stars, the distance is increasingly uncertain. Nevertheless, we are reasonably confident that Spica is at about 140 light years, Antares at about 550, and most distant among these seven is Deneb, perhaps 1,600 light years away.
The temperature of the region in the atmosphere of the star from which most radiation is coming ("effective temperature") is highest for Spica, some 26,000 degrees (Celsius), while the red Antares is coolest, only about 3,700 degrees.
The red supergiant Antares compensates for its low light output from unit surface area by its enormous size: its radius is probably about 420 times larger than the radius of the Sun; Deneb has about 100 solar radii, and Arcturus about 20. Smallest among the seven is Altair, whose radius exceeds the radius of the Sun by only 60%.
The true radiation output of each of the competitors must be calculated by properly evaluating, and then combining, all three factors. When we do this, then the absolute winner is Deneb, the star at the top of the "Northern Cross", which, to our eyes, appears fainter than the other five, and only a bit brighter than Regulus in Leo. Deneb wins by its large size combined with fairly high effective temperature. We estimate that it radiates as much as 64,000 Suns, and it is only its large distance from us that makes it rather inconspicuous in our sky. The huge red supergiant, Antares, radiates about half as much as Deneb. Third place belongs to Spica, which is rather small (about 4 solar radii only), but its high effective temperature contributes a lot to its high radiation output, which is more than 6,000 Suns.
The radiation output of the remaining four competitors is much more modest, and they appear bright to us mainly because of their proximity. Regulus, being fairly hot (13,000 degrees), shines like 250 Suns. Arcturus is a reddish giant (a radius of 19 solar radii but effective temperature lower than that of the Sun) and produces 130 solar luminosities. Vega is significantly hotter at the surface (9,600 degrees), but is rather small (2.5 solar radii), so it radiates as 48 Suns. And the loser is…Altair, with a radius only 60% larger than the radius of the Sun; thanks to its higher effective temperature, about 8,000 degrees, it still manages to radiate as much as 9 Suns. Its place among the stars of the first magnitude is mainly due to its relative proximity to us.
We have looked at seven stars only, and even from this small sample it is obvious that stars come in a surprising variety of sizes, temperatures, and distances. The Sun comes out of this competition as rather small and cool. Yet, on the coming summer days, you certainly will not wish to have it bigger and hotter than it is! In reality, it is both hotter and larger and brighter than the majority of stars around us. When watching the first magnitude stars, we are watching something like a competition of the top athletes: what they accomplish also does not represent us, the ordinary people!

Santa Monica 

Amateur Astronomy Club

Now in its eighteenth year, the Santa Monica Amateur Astronomy Club meets on the second Friday of every month at 7:30 p.m. at the Crosstroads School. In addition to guest lecturers, it has star parties, field trips and social gatherings. Its meetings are free and open to the public.