Royal Planets Rule the Evening Sky, the Waning Moon meets Mars, See Stars’ Colours, and more Meteors!
Seeing Colour in Stars
During evenings at this time of year you can see a variety of bright stars, each shining with a distinctive colour of starlight!
While all stars have intensely hot cores (fusing atoms together at millions of degrees under extremely high pressure), their surface temperatures are much cooler than that. (I use the term “surface” loosely. The round ball of the sun isn’t solid — the curved edge we see is just the radius where its plasma becomes opaque. Just as some clouds seem to have well defined edges, but we can fly straight through them.) Stars’ surface temperatures vary depending on their mass, age, and composition. And the temperature lends a unique spectral colour to their light that is visible using your unaided eyes if the star is bright enough. Cool stars are red and often dim, the hottest stars shine with a bright blue-white light, and the others, like our yellow Sun, fall between the two extremes.
After the sky darkens around 9 pm local time, head outside and face west. Turn your gaze to the upper half of the sky, and look for a matching pair of prominent stars oriented left-right and separated by only a few finger widths. Those are Castor and Pollux — the heads of the twins of Gemini. At first glance, they look the same. But closer scrutiny will reveal that Pollux, the star on the left, is a warmer, more yellowish colour than Castor — and it’s a bit brighter. Pollux is slightly cooler than our yellow sun, while Castor is hotter and whiter.
Three fist diameters to the lower right of Castor you’ll find bright Capella, another yellow star that is even closer to our sun’s temperature! Compare those three stars to bright Procyon, the star sitting two fist diameters to the lower left of Pollux. Procyon is a medium-hot yellow-white star. If your southwestern horizon is clear of obstructions, you also might be able to see very bright Sirius. It’s the same temperature as Castor, but it is much closer to Earth — 8.6 versus 51.6 light-years. Sirius also features flashes of other colours not related to its temperature, but due to its bright light being randomly refracted by our atmosphere — like the glints off a diamond.
Now swing around and look east. The most dominant star you will see is Arcturus. This very bright star (fourth brightest in the night sky) will be sitting in the lower half of the sky, somewhat below, and to the right of, the Big Dipper’s handle. Arcturus should have an obvious orange-ish tint to it due to its lower surface temperature.
Using these few bright sample stars, you should now be able to judge the relative tints, and therefore the temperatures, of many other stars in the spring sky. Check out Regulus high in the southern sky and Spica low in the southeastern sky. Once you have mastered the technique with unaided eyes, try using binoculars (or a telescope). If you unfocus the star into a fuzzy ball, its colour will become more obvious!
The Moon and Planets
Tonight (Sunday evening) brings us the full moon. The April full moon is called the Pink Moon, Sprouting Grass Moon, Egg Moon, or Fish Moon, after Mother Nature’s stirrings this time of year. And the April full moon always shines in or near the stars of Virgo (the Maiden). Recently, I presented a talk about the unique sights to see on the full moon. You can watch it on YouTube here and print out my list of targets here.
Tonight’s full moon will rise at 7 pm local time — give or take a bit, depending on your latitude on Earth. During the course of the night, the moon’s orbital motion will carry it towards Jupiter. At dawn on Monday, the two objects will be about to set in the western sky and separated by only 6° (your palm’s width held at arm’s length). When the moon rises on Monday evening, it will have shifted well to the left of Jupiter.
The moon will spend this week waning towards next Monday’s last quarter phase, when we will see it half illuminated on the left side — towards the pre-dawn sun. In the meantime, it will visit Saturn and Mars in the midnight-to-dawn southern sky. On Friday morning, the moon will sit to Saturn’s right. The next morning, it will hop east to sit between Mars and Saturn. And on Sunday, the last quarter moon will sit just two finger widths above Mars. Both objects will fit into the field of view of binoculars. The binocular field will also include the globular cluster Messier 75, which will be positioned 3° east of Mars.
Venus will continue to gleam in the western early evening sky this week until it sets, two hours after the sun — at about 10:45 pm local time. Tonight we’ll see the bright planet about a palm’s width to the left of the triangular face of Taurus (the Bull), and the bright reddish star Aldebaran. The rest of the week will see it climbing higher as the constellation sinks lower.
While queen Venus presides over the western sky, king Jupiter will be holding court in the east. This week, it will be rising just before 9 pm local time, will reach its highest elevation (about three fist diameters) above the southern horizon around 2 am local time, and then descend into the southwestern sky at sunrise. On Monday evening from 9 to 11 pm EDT time, Jupiter’s innermost moon Io and its little round black shadow will cross (or transit) Jupiter’s disk. A reasonable backyard telescope will show the black shadow, but a very good telescope is needed to see Io, too.
The Great Red Spot takes about three hours to cross Jupiter’s disk. But the planet’s 10-hour rotation period (i.e., its day) means that the spot is only observable from Earth every 2–3 nights. If you’d like to see the GRS, use a medium-sized telescope (or larger). You’ll have your best luck on evenings with steady air — when the stars are not twinkling too much. The best times to try this week are: Tuesday, May 1 at 12:55 am, Thursday, May 3 at 10:24 pm EDT, and Sunday, May 6 at 12:02 am. All times given are Eastern Daylight Time — adjust for your local time zone. Try to look within an hour before or after the times I’ve given.
The ringed planet Saturn will be rising in the east just before 1 am local time this week. In two weeks it will begin rising before midnight. You should be able to see its yellow-tinted point of light until about 6 am, when it will sit about 2.5 fist widths above the southern horizon. Reddish Mars will rise about an hour after Saturn, which places it 1.4 fist widths to the lower left of Saturn. Mars is continuing to brighten slowly as the Earth’s faster orbit brings us closer to the red planet. We will eventually pass it on the “inside track” in late July.
This week Mercury continues its trek across the eastern pre-dawn horizon. This appearance isn’t very good for observers in mid-northern latitudes. This morning (Sunday) it reached its widest separation west of the sun and peak visibility. Your best time to hunt for it this week will be between 5:30 and 5:45 am local time. If you live south of the equator, however, Mercury will be very easy to see for the next week or so.
Another Meteor Shower
The annual Eta Aquariid Meteor Shower, produced by material from Halley’s Comet, runs from Apr 19 to May 26 and peaks before dawn on Saturday morning. True Aquariids will appear to travel away from a radiant point in the sky in Aquarius (the Water-bearer), near the eastern horizon. This shower is better for folks in the Southern Hemisphere. Expect up to a few dozen meteors per hour, including some fireballs, near the peak. But the waning last quarter moon will degrade the sky for this shower.
Astronomy Skylights for this week (from April 29th, 2018) by Chris Vaughan.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.