Jupiter Meets Uranus in Twilight; Supernova Erupts in Nearby Spiral Galaxy

By: Bob King April 16, 2024 0

Jupiter aligns with distant Uranus before it exits the evening sky as a bright supernova flares in the southern galaxy NGC 3621.

Jupiter Uranus 12P comet — On April 14, Comet 12P/Pons-Brooks appeared 3.2° below Jupiter and Uranus 1.2° above it 75 minutes after sunset. Jupiter is still easy to see low in the western sky at dusk.
*Bob King*

Thanks to Comet 12P/Pons-Brooks, which makes its final appearance at dusk for Northern Hemisphere observers this week, many of us have kept a close eye on Jupiter. The gas giant has been an excellent guide to finding the periodic comet. On a trip to the eastern Pacific and Mexico to view the recent total solar eclipse, I eagerly awaited the planet's appearance every clear evening because it proved the perfect steppingstone to the fainter, fuzzier object.

Jupiter Uranus conjunction — Jupiter slides under Uranus between April 17–21. The animation shows the planets' positions and moons around 9 p.m. CDT on the dates shown. North is up.
*Stellarium*

As the comet sinks from view, Jupiter has one last, unofficial task before it departs the evening sky. For the next week or so it will guide observers to the planet Uranus. Some of you have probably noticed their proximity and already used the fifth planet to find the seventh. If not, this is your chance. Their separation will continue to tighten until they reach conjunction around 8 UT on April 20th. In the Americas they'll be closest at dusk on April 19th, separated by a hair more than 0.5°.

Start looking about 65–75 minutes after sunset very low in the western sky. The pair will hover just 5° to 7° high at that time, so you'll need at least 50-mm binoculars to spot 6th-magnitude Uranus. A small telescope will guarantee success and show both planets in the same low-to-medium-magnification field of view. Use this sunset calculator to help plan your observing session.

Last hurrah

The comet soon disappears in the twilight glow for northern skywatchers, while Jupiter transitions from dusk to dawn on May 18th. Southern Hemisphere observers have waited months to get their first look at this predominately northern object. And just in time! Comet 12P/Pons-Brooks should remain around magnitude 4–4.5 as it races toward an April 21st perihelion. Thereafter it's expected to fade even as its arc bends toward us — on June 2nd the comet will pass a relatively distant 1.5 a.u. (220 million kilometers) from Earth.

Supergiant supernova

As Jupiter recedes and night arrives, you can turn your telescope to the recently discovered Type II supernova SN 2024ggi in NGC 3621 located 3.2° southwest of 3.5-magntude Xi (ξ) Hydrae. The automated Asteroid Terrestrial-impact Last Alert System (ATLAS) survey flagged the initially very faint object 70.4″ east and 84.7″ south of the galaxy's center on April 11th. Given the galaxy's proximity to our own — a mere 22 million light-years away — hopes were high it would quickly brighten to within the range of amateur scopes. It has not disappointed. By April 15th the supernova had crested to around magnitude 11.5–12.0, easy enough to catch in a 6-inch scope from dark skies.

NGC 3621 locator — Finding the supernova host galaxy NGC 3621 is an easy hop from Epsilon (ε) Corvi through Xi (ξ)) Hydrae. The sky is shown for Des Moines, Iowa, (latitude 41.6° north) where the galaxy stands about 16° high at meridian crossing.
*Stellarium*

Before you rush out for a look, know that SN 2024ggi squats at declination –33°. From my observing site here at latitude 47° north that number initially gave me pause, but with the galaxy 10° above the southern horizon at culmination the supernova should still be within reach of an 8-inch or larger instrument. Just make sure you have an unobstructed view in that direction. Oh, and bring a kneepad if you're using a Dob. For tropical and Southern Hemisphere observers the supernova is pure candy.

Type II supernova artist's view — This artist's impression depicts SN 1993J in M81, a Type IIb supernova similar to SN 2024ggi. When fusion ceases in the core of the supernova progenitor star, gravity crushes the inner core into a dense sphere of neutrons about 20 kilometers across. Once formed, it resists further collapse. Infalling material crashes against the core then rebounds to produce a shock front that races through the star at some 50 million miles per hour (20,000 km/sec), tearing it apart in a massive explosion. The blue star at center (shown here) is the surviving companion star of the former supergiant in the SN 1993 system.
*NASA / ESA / G. Bacon / STSci*

Type II supernovae occur when a supergiant star runs out of fusible elements in its core. Without the pressure and heat produced by nuclear burning to resist gravity's now irresistible grasp, the star's core collapses in a matter of seconds. Shockwaves created when infalling material rebounds rips the star apart. Hot gases from the explosion form a rapidly expanding, luminous shell around the former core which either collapses to form a neutron star or — if its final mass is greater than 2 to 3 times solar — a black hole.

To keep tabs on the supernova's brightness, visit David Bishop's Latest Supernovae or the Supernovae Enthusiasts Group on Facebook. If possible, track down the galaxy and its new stellar beacon before full Moon. If clouds are a problem, evening skies will be moonless again starting around April 25th.