Last Stand For The Northern Cross And First Black Hole

The Northern Cross stands straight up and down in the lower third of the northwestern sky at nightfall (around 6:30 p.m.) in mid-January. To its lower right shines the brilliant star Vega. Cygnus X-1, found in 1964, was the first black hole discovered. Bob King

The Northern Cross is almost as beloved by beginning stargazers as Orion’s Belt or the North Star. It’s one of the better known asterisms, an easily recognizable pattern of stars within or shared by more than one constellation. The cross is the larger part of the more extended constellation Cygnus the Swan. Just re-envision the head (Deneb) as the swan’s tail, the crossbeam as outspread wings and the foot as the swan’s head, and you’ve got two completely different ways of looking at the figure.

Prefer a mythological swan to a cross? Here’s Cygnus from Bode’s Star Atlas

In mid-January, the cross / swan is long past its prime. Back in August it stood nearly overhead and tilted on its side, dragging the starry band of the Milky Way along for the ride. Now, it’s exiled to the west but makes one “last stand” before it dips below the horizon. To the cross’s lower right you might also catch sight of Vega.

Stars and constellations in the eastern sky at nightfall like Orion the Hunter are all on the rise and climb higher until they culminate (reach their highest point in the sky) when they stand due south. Then they move into the western sky and commence the process of setting. The dividing line between rising and setting is an imaginary semi-circle called the meridian. It begins at the due south point on the southern horizon, extends up through the overhead point and then drops down to the due north point on the northern horizon.

Material from the supergiant star HDE 226868 (left) is drawn out and spun into a disk by the powerful gravity of a black hole nearly 15 times the mass of the sun. The two objects are only about 18 million miles apart or 1/2 the distance Mercury is from the sun. ESA/Hubble

Rich in double stars and glowing gas clouds called nebulae the Northern Cross has plenty to offer skywatchers no matter what instrument you chose.  It also contains the enigmatic Cygnus X-1, the very first black hole discovered and one of the strongest sources of X-ray radiation, hence the X. Cygnus X-1 lies is a binary system 6,100 light years from Earth composed of the blue supergiant star HDE 226868 and a black hole 14.8 times as massive as the sun.

Use this chart to help you find the supergiant HDE 226868 and the invisible black hole it orbits every 5.6 days. The circle 1° across, about the field of view of a low power telescope. Bob King

Material from the supergiant star is funneled into the maw of the black hole. Friction created as the material whirls down the hole heats it to extreme temperatures, causing it to radiate tremendous amounts of energy that astronomers detect here on Earth. While no one can “see” the black hole in Cygnus X-1, you can spot HDE 226868, the star orbiting the hole that’s slowly being devoured alive. It’s magnitude 9 and visible in 50 mm binoculars from a dark sky or in a small 3-inch telescope from the suburbs.

Face northwest the next clear night and bid adieu to the Northern Cross and bon appetite to Cygnus X-1.

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