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Cosmic Challenge: Einstein's Cross

From an aesthetic perspective, the most perfect gravitational lens is Einstein's Cross, formed by the galaxy PGC 69457 (cross- cataloged as CGCG 378-15) and the quasar QSO 2237+0305 in Pegasus. PGC 69457 is also known informally as Huchra's Lens after its discoverer, John Huchra, professor of cosmology Harvard University. Current estimates place this small, otherwise unspectacular spiral galaxy at 400 million light years away. The quasar lurks far behind at an incredible distance of 8 billion light-years. Were it not for gravitational lensing, the quasar would remain hidden by the galaxy, as the two are nearly in-line as seen from Earth. But as it is, Huchra's lens fractures the ancient light from the quasar into four separate paths that slide around the galaxy just as water flows around a rock in a stream. The end result is not one, but four ghostly images of QSO 2237+0305 surrounding the nucleus of PGC 69457 in a practically perfect diamond pattern.

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Cosmic Challenge: North America Nebula (NGC 7000)

The North America Nebula (NGC 7000) is a large expanse of glowing hydrogen gas mixed with opaque clouds of cosmic dust just 3° east of Deneb [Alpha (α) Cygni] and 1° to the west of 4th-magnitude Xi (ξ) Cygni. Famous as one of the most luminous blue supergiants visible in the night sky, Deneb marks the tail of Cygnus the Swan, or if you prefer, the top of the Northern Cross asterism.

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Cosmic Challenge: Campbell's Hydrogen Star

American astronomer William Wallace Campbell spotted this unusual star-like object through a visual spectroscope at Lick Observatory in 1893. He could tell immediately from its spectrum that, despite its stellar appearance, he was not seeing an ordinary star at all. Instead, he had spotted an uncharted planetary nebula.

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Cosmic Challenge: Dissecting M101

Just spotting the gigantic Pinwheel Galaxy, M101, can sometimes be challenging enough. Its low surface brightness can drive suburban observers crazy, especially when we see photographs that show it so big and bright, or that it is listed as 8th magnitude. It all comes down to surface brightness, or more accurately, lack of surface brightness. Seeing the dim glow of the galaxy's small core, or the even dimmer glimmer of the surrounding spiral arms, can take a concerted effort. But with time and patience, M101 is visible, with difficulty, through 50-mm binoculars even given a suburban sky with a naked-eye limiting magnitude of perhaps 4.5.

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Cosmic Challenge: Glimpsing Vesta

Vesta turns out to be more like a mini-planet than like the chunks of rock most think of as asteroids. Dawn's measurements of the gravity field provided good evidence that Vesta's interior is separated into layers, much like Earth did as the planet was forming. Vesta's dense core - apparently once molten, but now solidified - is composed principally of iron and nickel, just like Earth's. Estimates place it at 125 to 150 miles (200 to 250 kilometers) across. Surrounding that is the mantle, which in turn is covered by the veneer of the crust, about 12 miles (20 kilometers) thick. It is now believed that early on Vesta was likely still accumulating material to become a full-fledged planet when Jupiter's immense gravity intervened, putting a stop to that. As a result, when we look at Vesta, many believe that we are seeing a protoplanet frozen in time.

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