Arcturus – Cosmic Reflections

Hidden Wonders of the Southern Sky

Here in southern Arizona, we can theoretically see 92.4% of the celestial sphere. I say “theoretically” because atmospheric extinction, light pollution, local topography, and obstructions limit the amount of the celestial sphere that we can see well. Also, far southern objects (down to δ = -58° at φ = 32° N) spend very little time above our horizon each day.

Practically speaking, then, we see somewhat less than 92% of all that there is to see from spaceship Earth.

Percent of the Celestial Sphere Visible

$\% = 50\left [ 1-sin\left ( \left|\varphi \right| -90^{\circ}\right ) \right ]$

where |φ| is the absolute value of your latitude in degrees

What are the most prominent objects we are missing, and what objects that we can see are they closest to?

Alpha Centauri

Never visible north of latitude 27° N, the nearest star system beyond our solar system is Alpha Centauri. Alpha Centauri A & B are bright stars, having a visual magnitude of 0.0 and +1.3, respectively, and in 2023 they are separated by just 8 arcseconds, about 1/4 of the angular separation between Albireo A & B. While Alpha Centauri A & B—which orbit each other once every 79.8 years—lie just 4.36 ly away, a faint red dwarf companion, Proxima Centauri (shining at magnitude +11.1), is even closer at 4.24 light years. It is not yet known whether Proxima Centauri, discovered in 1915, is gravitationally bound to Alpha Centauri A & B, or just presently passing through the neighborhood. Proxima is a full 2.2° away (over four moon-widths) from Alpha Centauri A & B.

When Arcturus (α Boo) and Zubenelgenubi (α Lib) are crossing our celestial meridian, so are Alpha & Proxima Centauri below the southern horizon.

Large Magellanic Cloud

The Large Magellanic Cloud (LMC), the largest satellite galaxy of our Milky Way galaxy and easily visible to the unaided eye, lies directly below our southern horizon when Rigel has crossed the meridian and Bellatrix is preparing to do so.

Small Magellanic Cloud

The Small Magellanic Cloud (SMC), the second-largest satellite galaxy of the mighty Milky Way lies underneath our southern horizon when M31, the Great Andromeda Galaxy, crosses the meridian near the zenith.

47 Tucanae

The 2nd brightest globular cluster in the sky (after Omega Centauri) is impressive 47 Tucanae. It is just 2.3° west and a little north of the Small Magellanic Cloud, so crosses the meridian below our horizon just as M31 is nearing the meridian.

Eta Carinae Nebula

Four times larger and brighter than the Orion Nebula, NGC 3372, the Eta Carinae Nebula, is a spectacular star-forming region containing a supermassive (130 – 180 M_☉) binary star (Eta Carinae) that may go supernova at any time. When Leo the Lion is straddling the meridian, the Eta Carinae Nebula sneaks across as well.

Any other spectacular objects I should be including that are south of declination -58°? If so, please post a comment here.

Lovely Coma Berenices

One of the special joys of getting out under a dark rural sky this time of year is seeing the gossamer beauty of the surprisingly expansive star cluster called Melotte 111, also known as the Coma star cluster. Mel 111 makes up a large part of the constellation Coma Berenices, “Berenice’s Hair”. This constellation, which entertains the North Galactic Pole as well as a gaggle of galaxies, can be found about midway between Denebola (some call the Coma star cluster the end of the “tail” of Leo the Lion) and Arcturus, as well as midway between Spica and the Big Dipper. Coma Berenices is transiting the meridian this week as evening twilight ends. At a distance of just 284 light years, the Coma star cluster is the third nearest star cluster to us, surpassed only by the open cluster remnant Collinder 285—the Ursa Major association (80 ly)—and the Hyades (153 ly).

Brightest Event Ever Observed

On June 14, 2015, perhaps the intrinsically brightest event ever recorded was detected at or near the center of the obscure galaxy APMUKS(BJ) B215839.70−615403.9 in the southern constellation Indus, at a luminosity distance of about 3.8 billion light years.

ASASSN-15lh (All–Sky Automated Survey for SuperNovae), also designated SN 2015L, is located at α₂₀₀₀=22^h02^m15.45^s, δ₂₀₀₀=-61° 39′ 34.6″ and is thought to be a super-luminous supernova—sometimes called a hypernova—but other interpretations are still in play.

Let’s put the brightness of SN 2015L in context. Peaking at an absolute visual magnitude of -24.925 (which would be its apparent visual magnitude at the standard distance of 10 parsecs), SN 2015L would shine as bright as the Sun in our sky if it were 14 light years away—about the distance to van Maanen’s Star, the nearest solitary white dwarf. SN 2015L would be as bright as the full moon if it were at a distance of 8,921 light years. SN 2015L would be as bright as the planet Venus if it were at a distance of 333,000 light years. Since the visible part of our galaxy is only about 100,000 ly across, had this supernova occurred anywhere in our galaxy, it would have been brighter than Venus. If SN 2015L had occurred in M31, the Andromeda Galaxy, 2.5 million light years away, it would take its place (albeit temporarily) as the third brightest star in the night sky (-0.47^m), after Sirius (-1.44^m) and Canopus (-0.62^m), but brighter than Alpha Centauri (-0.27^m) and Arcturus (-0.05^m).

The Open Supernova Catalog (Guillochon et al. 2017) lists three events that were possibly intrinsically brighter than SN 2015L. Two events were afterglows of gamma ray bursts GRB 81007 and GRB 30329: SN 2008hw at -25.014^m and SN 2003dh at -26.823^m, respectively. And the other event was the first supernova detected by the Gaia astrometric spacecraft, Gaia 14aaa, 500 Mly distant, shining perhaps as brightly as -27.1^m.

References
Chatzopoulos E., Wheeler J. C., Vinko J., et al., 2016, ApJ, 828, 94
Dong S., Shappee B. J., Prieto J. L., Jha S. W., et al., 2016, Science, 351, 257
Guillochon J., Parrent J., Kelley L. Z., Margutti R., 2017, ApJ, 835, 64