Skywatcher's Guide: April and May 2024

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• Stars and Constellations
• Solar System
• Calendar of Night Sky Events
• Deep Sky
• Frequently Asked Questions

Stars and Constellations

In April, the spring sky is now prominent in the east, along with the familiar Big Dipper.  The Big Dipper is high in the northeast, and the two stars at the end of the bowl can be used to find Polaris, our north star.  Also, the handle can be used to "arc to Arcturus", a bright star in the constellation Boötes. Next, Leo the lion is very high in the east, approaching the middle of the sky now, with the constellation Virgo just below.  The winter constellations are also visible, now in the west.  Taurus the bull is getting lower in the west, near Orion the hunter to the west-southwest.  Canis Major (the big dog), along with the bright star Sirius, is also getting low in the southwest.  Gemini the twins is a little higher in the west, along with Auriga the charioteer in the west-northwest and Canis Minor (the little dog) in the southwest.  The winter Milky Way is now getting lower in the west as well.  Far to the south we can see some constellations making up part of Jason's Argo Navis, which we only get to see briefly since it is so far south.  Finally, there is still a small portion of the fall sky still visible just for the first hour or so of the night.  Cassiopeia the queen is in the north-northwest, and Perseus the hero is in the northwest.

In May, the winter constellations are lower in the west, and Orion, Taurus, and Canis Major are already in the process of setting at the beginning of the night, along with Cassiopeia and Perseus to the northwest.  The spring constellations are higher now, and we can see the rest that weren't up this time last month.  Hercules is below Boötes in the east-northeast.  Some of the summer constellations are also getting ready to come up and will be visible a few hours after sunset.

Interesting Stars Visible in April and May (from 7-10 pm)

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Solar System

Mercury passes behind the Sun in mid-April and will be visible in the morning sky for the first half of May.

Venus is now lost in the glare of the Sun.

Mars stays close to the eastern horizon before sunrise but gradually comes up earlier and earlier. It is moving through Aquarius and Pisces.

Jupiter is visible in the west after sunset during April but will be passing behind the Sun in mid-May.

Saturn stays in Aquarius, rising a little before sunrise in early April, getting higher and higher every day.

Jupiter Great Red Spot Transits in April and May (from 7-10 pm)

Note: The GRS is visible on the disk of Jupiter for 50 minutes before and after meridian transit time.

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Calendar of Night Sky Events

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Deep Sky

The winter Milky Way is now getting low in the west, but there are still several interesting objects we can see here.  The Pleiades (Seven Sisters, M45) is close to the west-northwest horizon right next to the Hyades (C41), which makes the face of Taurus the bull.  The constellation Auriga is a little bit higher, where we can see M36, M37, and M38, which are visible with binoculars.  The Double Cluster (C14) in Perseus is low in the north-northwest.  Higher in the sky we have the Beehive (Praesepe, M44) in Cancer the crab and Coma Berenices (Bernice's Hair) near the tail of Leo.

We are now beginning to see some globular clusters coming up in the east.  M3 is towards the east, in the constellation Boötes.  Nearby, the famous Hercules Globular (M13) is low to the east-northeast.

For nebulae, we have the spectacular Orion Nebula (M42) now now getting low to the west-southwest. This is the closest star-forming region to our solar system.  We also have some good planetary nebulae, which come from dying stars.  The Eskimo (C39) in Gemini is high in the west, the Owl (M97) in Ursa Major is high in the northeast, and the Ghost of Jupiter (C59) is to the south in the constellation Hydra.

And now the galaxies:  In Ursa Major to the north we have Bode's Galaxy (M81) and the Cigar Galaxy (M82), close enough to be seen together in a low-power telescope.  Nearby in the constellation Canes Venatici we have the Whirlpool (M51), which is a pair of colliding galaxies.  The Pinwheel Galaxy (M101) is also nearby near the handle of the Big Dipper.  Then the Sombrero Galaxy (M104) is in the southeast in the constellation Virgo.

Interesting Deep Sky Objects to Observe during April and May (from 7-10 pm)

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Frequently Asked Questions

How big can a planet get before it becomes a star?

You may have heard Jupiter described as a failed star, but how big would Jupiter have to get to actually become a star?

In 2006 the International Astronomical Union came up with a definition that separates planets from dwarf planets and smaller objects like asteroids and comets.  However, there still is no clear delineation between a star and a planet. We know generally what a star looks like and what a planet looks like, but there’s kind of a blurry line separating the two.  Caught in the middle is something called a brown dwarf, which shares some qualities with both planets and stars.

Brown dwarfs have masses between 13 and 80 times that of Jupiter.  In that range, there is enough pressure in the core to fuse deuterium (²H, aka heavy hydrogen), but not regular hydrogen (¹H, aka protium).  They therefore generate light and heat, although not very much.  Their temperatures are between 500 K and 3000 K, and they emit mostly infrared light and a little bit of visible red light.  Often brown dwarfs don’t have enough deuterium to power them for very long, so they eventually stop fusing and cool down.

Interestingly, all brown dwarfs have roughly the same radius, about equal to the radius of Jupiter.  As more and more material gathers together, the gravity squeezes it together more strongly, so the size actually doesn’t increase very much.  This is another reason the line between planets and stars is hard to distinguish.  Many brown dwarfs appear in exoplanet catalogs as well as star catalogs.

Some astronomers propose to use the object’s manner of formation to determine whether it is a star or a planet.  Stars form from the gravitational collapse of gas in a nebula, while planets form from debris in a protoplanetary disk.  But how a particular object formed is not always easy to determine, and there will still be overlap between the biggest planets and the smallest stars.

If you have any questions you'd like me to answer in the next issue of SWG, please let me know.  I'm also happy to take suggestions or comments, and also pictures if you'd like to send them.  Happy viewing!

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