A Nebula that Extends its Hand into Space

The Gum Nebula is an emission nebula almost 1400 light-years away. It’s home to an object known as “God’s Hand” among the faithful. The rest of us call it CG 4.

Many objects in space take on fascinating, ethereal shapes straight out of someone’s psychedelic fantasy. CG4 is definitely ethereal and extraordinary, but it’s also a little more prosaic. It looks like a hand extending into space.

The Dark Energy Camera (DECam) on the NSF’s Víctor M. Blanco 4-meter Telescope captured the image. DECam’s primary job is to survey hundreds of millions of galaxies in its study of dark energy. But it’s also a general-purpose instrument used for other scientific endeavours.

CG 4 is called a cometary globule because of its appearance. But it’s actually a star-forming region. It has a head that’s about 1.5 light-years in diameter and a tail that’s about 8 light-years long. The head is dense and opaque and is lit up by a nearby star. The globule is surrounded by a diffuse red glow, emissions from ionized hydrogen.

This excerpt shows a close-up of CG 4. The hand looks like it’s about to grasp an edge-on spiral galaxy named ESO 257-19 (PGC 21338). But the galaxy is more than a hundred million light-years beyond CG 4. Only a chance alignment makes it seem close. Near the head of the cometary globule are two young stellar objects (YSOs). They’re stars in their early stage of evolution before they become main-sequence stars. Image Credits: Credit: CTIO/NOIRLab/DOE/NSF/AURA
Image Processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), D. de Martin & M. Zamani (NSF’s NOIRLab)

There are lots of cometary globules in the Milky Way. They’re a sub-class of objects called Bok globules, after astronomer Bart Bok, who discovered them. Both types of globules are dark nebulae, molecular clouds so dense they block optical light. Astronomers aren’t absolutely certain how cometary globules get their shape.

But they do know what’s happening to them.

The red glow surrounding CG 4 is ionized hydrogen lit up by radiation from nearby hot, massive stars. That same radiation is eroding CG 4 away. Since the globule is denser than its surroundings, it’s resisting diffusion. It still contains enough gas and dust to form several new stars about as massive as the Sun.

In this zoom-in, the hand looks more like the mouth of the Shai-Hulud, reaching out into space to destroy the approaching Sardaukar. Image Credit: CTIO/NOIRLab/DOE/NSF/AURA. Image Processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), D. de Martin & M. Zamani (NSF’s NOIRLab)

Even though there are many of these globules in the Milky Way, the majority of them are in the Gum Nebula. Scientists know of 31 other globules in the nebula. This one’s called CG 4 (Cometary Globule 4) because they’re all numbered.

This image shows three of the 32 CGs in the Gum Nebula: CG 30, 31, and 8. Image Credit: By Legacy Surveys / D.Lang (Perimeter Institute) & Meli Thev – Own work, CC BY 4.0, https://ift.tt/NEzYuQF

The Gum Nebula is likely the remnant of a huge supernova explosion, and that could be the reason the globules have their unique shape. They may have originally been spherical nebula like the Ring Nebula. But a powerful supernova explosion about one million years ago stretched them into their long, comet-like forms.

The James Webb Space Telescope captured this image of the Southern Ring Nebula, or NGC 3132, with its NIRCAM instrument. Cometary globules could’ve started out as ring-shaped nebulae before being deformed by supernova explosions. Image Credit: By Image: NASA/ESA/CSA/Space Telescope Science Institute. Public Domain

Astronomers also suggest another reason for their shape. Nearby hot, massive stars exert radiation pressure on the globules, and their stellar wind also slams into them. In the Gum Nebula, their tails point away from the Vela Supernova Remnant and the pulsar that sits in its centre. Since the Vela Pulsar is a spinning neutron star, it’s possible that its winds and radiation pressure are shaping CG 4.

Whatever its cause, the Hand of God is a visually intriguing object. If you really want to lose yourself in this amazing nebula, download the TIFF file here.

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