Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way. About 50 million light-years distant in the northern constellation Pegasus, NGC 7331 was recognized early on as a spiral nebula and is actually one of the brighter galaxies not included in Charles Messier’s famous 18th century catalog. Since the galaxy’s disk is inclined to our line-of-sight, long telescopic exposures often result in an image that evokes a strong sense of depth. In this Hubble Space Telescope close-up, the galaxy’s magnificent spiral arms feature dark obscuring dust lanes, bright bluish clusters of massive young stars, and the telltale reddish glow of active star forming regions. The bright yellowish central regions harbor populations of older, cooler stars. Like the Milky Way, a supermassive black hole lies at the core of of spiral galaxy NGC 7331.
What’s happening to galaxy NGC 474? The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple through the galactic giant. Regardless of the actual cause, the featured image dramatically highlights the increasing consensus that at least some elliptical galaxies have formed in the recent past, and that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with — and accretions of — smaller nearby galaxies. The halo of our own Milky Way Galaxy is one example of such unexpected complexity. NGC 474 spans about 250,000 light years and lies about 100 million light years distant toward the constellation of the Fish (Pisces).
An unusual type of solar eclipse occurred in 2012. Usually it is the Earth’s Moon that eclipses the Sun. That year, most unusually, the planet Venus took a turn. Like a solar eclipse by the Moon, the phase of Venus became a continually thinner crescent as Venus became increasingly better aligned with the Sun. Eventually the alignment became perfect and the phase of Venus dropped to zero. The dark spot of Venus crossed our parent star. The situation could technically be labeled a Venusian annular eclipse with an extraordinarily large ring of fire. Pictured here during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory, with the dark region toward the right corresponding to a coronal hole. Hours later, as Venus continued in its orbit, a slight crescent phase appeared again. The next Venusian transit across the Sun will occur in 2117.
The Earth’s dark umbral shadow is shaped like a cone extending into space. Of course its circular cross section at the distance of the Moon is more easily seen during a lunar eclipse. In fact, in this composite telephoto image from Earth’s night side on January 31, the Earth’s shadow has taken on a reddish tinge. The extent of the shadow along the lunar orbit is illustrated by aligning three frames taken just before the start, near the middle of, and just after the end of the total eclipse phase that lasted about 76 minutes. At the upper right and more easily seen during the eclipse’s darker total phase is M44, one of the closest large star clusters. A mere 600 light-years away, M44 is also known as the Praesepe or the Beehive Cluster.
Near the closest point in its orbit, the second Full Moon of the month occurred on January 31. So did the first Total Lunar Eclipse of 2018, as the Moon slid through planet Earth’s shadow. In a postcard from planet Earth, this telescopic snapshot captures the totally eclipsed Moon as it set above the western horizon and the Chiricahua Mountains of southern Arizona. The Moon’s evocative reddened hue is due to sunlight scattered into the shadow. Still, the planet’s shadow visibly grows darker near the center, toward the top of the lunar disk.
Sixty years ago, on January 31, 1958, the First Explorer was successfully launched by the Army Ballistic Missile Agency on a Jupiter-C rocket. Inaugurating the era of space exploration for the United States, Explorer I was a thirty pound satellite that carried instruments to measure temperatures, and micrometeorite impacts, along with an experiment designed by James A. Van Allen to measure the density of electrons and ions in space. The measurements made by Van Allen’s experiment led to an unexpected and then startling discovery of two earth-encircling belts of high energy electrons and ions trapped in the magnetosphere. Now known as the Van Allen Radiation belts, the regions are located in the inner magnetosphere, beyond low Earth orbit. Explorer I ceased transmitting on February 28, 1958, but remained in orbit until March of 1970.
Will the spider ever catch the fly? Not if both are large emission nebulas toward the constellation of the Charioteer (Auriga). The spider-shaped gas cloud on the left is actually an emission nebula labelled IC 417, while the smaller fly-shaped cloud on the right is dubbed NGC 1931 and is both an emission nebula and a reflection nebula. About 10,000 light-years distant, both nebulas harbor young, open star clusters. For scale, the more compact NGC 1931 (Fly) is about 10 light-years across.