Skywatcher's Guide: April and May 2022

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Navigation

• 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 (during observatory hours)

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Solar System

Mercury will be passing behind the Sun in early April, but will be visible in the evening sky later in the month and in early May.  It then passes between us and the Sun by the end of May.

Venus is bright and high in the southeastern sky before sunrise.

Mars rises earlier and earlier each morning before sunrise, moving through Capricornus, Aquarius, and Pisces.

Jupiter is emerging from the glare of the Sun just before sunrise during April and will be more easily visible as we get into May.  It is moving from Aquarius into Pisces.

Saturn stays in Capricornus, rising in the early morning during April and getting higher and higher every day.

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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 (during observatory hours)

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Frequently Asked Questions

Why are galaxies different colors?

Galaxies are all made of the same stuff, so they should be roughly the same color, right?  Well, there are actually a few different factors that can affect the color of a galaxy.

First, when looking through a telescope with your eye, all galaxies look pretty much grey.  That's because they don't give off enough light to trigger the cones in your eye to be able to tell what color they are.  So we have to rely on cameras to gather more light and determine their color.  This process involves using filters to collect one color of light at a time and then combining them into a single image.  This allows us to enhance certain colors or even include colors our eyes cannot see in order to make a beautiful image.   So part of the answer to our question is just how the image was created.  But it's not the whole answer.

Scientists can objectively determine the color of an object by comparing the amount of red vs blue light that the object gives off.  So some galaxies are in fact bluer and some are redder.  This has to do with the galaxy's age, density, composition, and history.  Older galaxies typically have fewer blue stars since they burn out more quickly compared to red stars, and therefore older galaxies tend to be more reddish.  Next, a compact galaxy can create a lot more blue stars than a sparse galaxy, so color can also be related to density.  The presence of dust can also make stars and therefore the galaxies they are in appear redder. And finally, when galaxies collide with each other, this can trigger new star formation and more blue stars than would have been there otherwise.

One other thing to mention is that galaxies are usually not a uniform color throughout, so there may be different factors at play in different regions of an individual galaxy.

One final factor that affects a galaxy's apparent color is its motion relative to Earth and the Milky Way.  Due to the expansion of the universe, most galaxies are moving away from us and exhibit what's called a redshift, meaning they appear redder than they would if they were stationary relative to us.  The farther away they are, the faster they appear to recede and the redder they appear.  Only some of the very nearest galaxies are moving towards us and exhibit a blueshift.

In summary, there are a lot of different reasons why galaxies might be different colors.  There are trillions or even quadrillions of galaxies in our visible universe, so there is no end to the variations we can see.

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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