Tuesday, September 27

North America Nebula is a true likeness – Astronomy Now

A widefield view of the North America Nebula and the Pelican Nebula. Image: Panagiotis Andreou.

The North America Nebula (NGC 7000/Caldwell 20) in Cygnus is a fantastic emission nebula, an iconic object providing one of the highlights of the summer sky. These days amateurs like to give a name to deep-sky objects and many monikers leave one head-scratching, trying to match a figure or pattern to the name. The North America Nebula is different: it really does bear an uncanny resemblance to the North American continent! It’s also a great imaging target and can be seen through a pair of binoculars under a dark sky.

The North America Nebula is located in Cygnus, a few degrees east of Deneb (alpha [a] Cygni). AN graphic by Greg Smye-Rumsby.

How to observe

The familiar form of Cygnus, the Swan, rides high on August nights, with its easily seen ‘Northern Cross’ asterism of its brightest star, including mighty Deneb (alpha [α] Cygni). The North America Nebula sits handily just over three degrees east of magnitude +1.2 Deneb. It’s a huge emission nebula, a vast H-II region that’s part of the same interstellar cloud of ionised gas as the neighbouring Pelican Nebula (IC 5070), which lies off the eastern seaboard. Together, they are formally known as Sh2-117 (Sharpless) and are separated by a dark area of obscuring dust. Emission nebulae such as these are so-called because they emit their own light; radiation from hot, massive stars embedded within are exciting the nebula’s atoms and molecules, ionising them and causing them to glow in familiar colours, such as the deep-red of hydrogen-alpha. 

Observations of the nebula (specifically hundreds of its embedded stars) by the Gaia spacecraft in 2020 refined the previously uncertain distance to Sh2-117, pinning it down to around 2,500 light years, over 700 light years further away than thought. With the revised information, astronomers now believe the whole nebula physically spans around 140 light years. 

A zoomed-in view of the eastern side of the North America Nebula and the Pelican Nebula. Image: Terry Hancock.

On the sky, the North America Nebula spans 120 × 100 arcminutes – that’s about four full-Moon widths – making it a prime binocular target, provided the night is transparent and moonless and your observing site is free from major sources of light pollution. The ‘Gulf of Mexico’ region and the ‘Wall’, the area that resembles Mexico and Central America, should be the easiest part of the nebula to spot at first glance. The Pelican Nebula covers about 60 x 50  arcminutes.

Telescopically, a wide field is best – you need to encompass about three degrees of sky – so a refractor of 75–100mm (~three- to four-inches) aperture and 500mm focal length are well suited. Employ a wide-field eyepiece giving a low magnification of about 20× and an O-III or UHC filter to help to improve the contrast. A small- to moderate-aperture telescope should pick out the tenth-magnitude open cluster NGC 6997; American observers say it’s located on the border between the states of Ohio and West Virginia. A 120mm (four-inch) telescope should reveal around ten stars across an area of about 10 arcminutes, while through a 250mm (10-inch) aperture about 40 stars should pop into view.

The Cygnus Wall and the Gulf of Mexico regions. Image: Kfir Simon.

source: astronomynow.com