What’s Up: Mercury at Sunset and Nova in Cassiopeia

May 14, 2021 | Daily Space, Mercury, Sky Watching, Stars

What’s Up: Mercury at Sunset and Nova in Cassiopeia
CREDIT: Pamela Gay/Stellarium

All right. We’re going to make a confession. Pamela and I have never really seen Mercury with our own eyes in the night sky. It’s a hard little planet to catch. It’s always ridiculously close to the Sun and either sets just before the Sun or rises just after and gets lost in the daylight. If it’s not up at sunset, I’m probably never going to be up early enough to catch it.

But now is our chance. Mercury is actually going to be visible multiple times this year, and it will reach its easternmost elongation – where it is farthest from the Sun in the sky – on May 17. You’ll still have to go out at dusk, but if you do that, and you look toward the west just above the setting Sun, you should be able to barely make out bright Venus in the afterglow and tiny Mercury above. Both planets will quickly set behind the Sun, so make sure you get out and look on the 17th when Mercury is easiest to see this month.

If you do miss the tiny planet, you can try again in the morning of July 4. After that, you’ll have to wait for sunset on September 14. As we said, this planet is a difficult one to catch sight of.

Last month, we brought you news of three novae that were visible with small telescopes. Now, one of those novae has unexpectedly brightened and is now visible to the unaided eye. Nova Cassiopeiae 2021, which was originally named V1405 Cas, was discovered at magnitude 9.6 in March by amateur astronomer Yuji Nakamura. It brightened to magnitude 8.0 and stayed there for a month, and then brightened again to 7.5.

IMAGE: This map shows the western half of the W and a suggested path (yellow) to the nova. North is up. I included the magnitudes of several stars so you can judge the nova’s changing brightness. For example, if it’s halfway between 5.0 and 5.6, then its magnitude would be 5.3. Record your observations to keep a record of its ups and downs over the coming weeks and months. CREDIT: Bob King/Stellarium

Suddenly, last week, the nova jumped up abruptly in brightness, crossing the line of human visibility at 6.0 magnitude and is currently at least a 5.5 and still brightening. You have to look at it with an averted eye, where you try to glimpse it in your peripheral vision, but you can see it without binoculars now. However, binoculars will make the viewing easier. The best time to view this nova is before dawn when it’s highest, and you have to be able to see the constellation of Cassiopeia in your sky.

Sky and Telescope explains how to find this nova: Start at 2nd-magnitude Caph, located at the western end of the W, and use the brighter stars and homemade triangle asterisms shown on the map as steppingstones to the nova. Just south of the nova look for a bejeweled, fuzzy patch of light, the bright star cluster M52.

We’ll have a link to the article and finding charts on our website, DailySpace.org.

Keep in mind that this is not a supernova. This is not a star dying. This is a pair of stars, a white dwarf and a Sun-like companion, and the white dwarf is stripping hydrogen gas off the larger star. This infalling material spirals around the white dwarf then gets heated and compacted by the star’s immense gravity. The hydrogen starts to fuse, and what we see is the release of all the energy in the ensuing explosion. Both stars go on to live another day.

So get out and check out these two events, one at sunset and one at sunrise. They’ll both be worth the effort.

More Information

Swimming up sky and up river (EarthSky.org)

Nova in Cassiopeia Brightens Suddenly (Sky & Telescope)


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