Astro Bob: Dwarf planet Ceres photobombs galaxy M100

On Sunday night, March 26, Ceres smiles for the camera right in front of a galaxy 55 million light-years away.

Ceres and M100
On Sunday night, March 26, the dwarf planet Ceres will pass directly in front of the spiral galaxy M100 in the constellation of Coma Berenices. Ceres is bright enough to see in binoculars, but the 9th magnitude galaxy will require a 6-inch or larger telescope.
Contributed / Stellarium with additions by Bob King

There are only five dwarf planets โ€” Pluto, Eres, Ceres, Makemake and Haumea. Ceres is the brightest, closest and the only one you can see in binoculars. It's also the largest asteroid in the main asteroid belt with a diameter of 296 miles (476 kilometers). If the Earth were the size of a nickel, Ceres would be a poppy seed.

The International Astronomical Union created the category of dwarf planets in 2006 in response to the ever-increasing number of Pluto-like asteroids discovered in the outer solar system in the 1990s and early 2000s. Rather than call the former planet just another asteroid, the group downgraded it to purgatory status as a dwarf planet. Dwarf planets orbit the Sun and are large enough to mold themselves into spheres through self-gravitation but lack the gravitational dominance of the more massive planets.

Late March Leo
To find Ceres first start by locating the bright constellation Leo the Lion. The easiest part is the lion's head, shaped like a backwards question mark with the bright star Regulus at the bottom. From there, slide a little more than two fists to the lower left to Denebola, which marks the lion's tail. Once you've found Denebola use the detailed maps (below) to navigate to Ceres.
Contributed / Stellarium

Ceres is easily visible in a pair of 7x35, 8x42 or 10x50 binoculars even under moderately light-polluted skies. It looks exactly like a star but instead of sitting in the same spot every night, it moves! As the dwarf planet orbits the sun this season it slowly creeps westward across the constellation Coma Berenices behind the tail of Leo the Lion. You can actually see its movement by checking back whenever it's clear out.

Coma Berenices is rich in galaxies. Vast beyond imagining and chock full of stars, planets and nebulae, galaxies are the coinage of the universe. Astronomers estimate there are at least 2 trillion of them out there. The Milky Way Galaxy โ€” the one we live in โ€” is spiral-shaped and contains between 250 and 400 billion stars including our favorite, the sun.

Ceres and M100
Ceres approaches the spiral galaxy M100 on March 25, 2023. The galaxy's name is from a catalog of deep sky objects compiled by 18th century French astronomer and comet hunter Charles Messier โ€” hence the "M" followed by a number.
Contributed / Eliot Herman

M100 is one of the brighter galaxies and like our own has spiral arms that wrap around it core. Located 55 million light-years away, light reaching our eyes tonight left the galaxy during the Eocene epoch, when the first modern mammals evolved including primates, whales and elephants. Size-wise it's about as big as our own galaxy with a diameter of 107,000 light-years.


Ceres binocular map
Once you've found Denebola take your binoculars (or telescope) and slide east to find the small T-shaped pattern of stars. Use it as your anchor to find and track Ceres in the coming nights. Positions are labeled every three nights.
Contributed / Stellarium with additions by Bob King

Through a telescope the galaxy looks like a round, fuzzy disk with a brighter center. My 15-inch reflector faintly reveals the two most prominent spiral arms that show up so beautifully in photos. If you have a scope, Sunday night (March 26) presents a special opportunity to watch Ceres pass directly in front of M100.

Leo and Ceres
I photographed Leo with Ceres on March 24, 2023 so you can get a sense of its stellar appearance. The dwarf planet is at opposition this month when it's closest to the Earth for the year. It shines at magnitude 7.0, one magnitude fainter than the dimmest naked-eye stars.
Contributed / Bob King

A bright asteroid transiting a bright galaxy is an uncommon occurrence, so I'm really looking forward to seeing this. For a few hours Ceres will mimic a dazzling supernova (yes, supernovae also look exactly like stars). If you observe M100 as soon as it gets dark and then return for a second look a couple hours later, you'll see that the "supernova" has shimmied along to the west. After 1 a.m. Central Time it parts ways with the galactic giant.

Italian astronomer Giuseppe Piazzi discovered the asteroid (the first known) on Jan. 1, 1801and named it for Ceres, the goddess of the harvest. The word "cereal," made from harvested grains, derives from the same. At the time, it was considered and called a planet until astronomers discovered many more like it. Then, similar to what happened with Pluto, it was downgraded to asteroid status.

Despite Ceres' battered appearance and lack of an atmosphere, it's rich in water, clay and carbonate minerals and may potentially harbor primitive life forms such as bacteria. Daytime surface temperatures there climb as high as -28ยฐ F (-33ยฐ C). No problem. We've survived that and worse here in Duluth, Minnesota. To be fair it does drop down to 225 below (-143ยฐ C) at night.

Whatever the weather is at your place in the coming nights, grab a pair of binoculars or a telescope and have a look at the multiple personality world of Ceres known by many names โ€” planet, asteroid, dwarf planet and now, lookalike supernova.

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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