The galaxy HCM-6A, shown here, is stretched and magnified by the effect of gravitational lensing by the foreground cluster of galaxies in front of it: cluster Abell 370. In 2002, the discovery of this galaxy, at a redshift of z=6.56 and a distance of ~28 billion light-years, took the record back for galaxies from quasars, and quasars have never held the record since. (Credit: J.-P. Kneib & P. Natarajan, Astron. Astrophys. Rev., 2011)
Beyond the planets, stars, and Milky Way lie ultra-distant objects: galaxies and quasars. Here’s how far back we’ve seen throughout history.
The night sky showcases untold astronomical riches.
Behind the dome of a series of European Southern Observatory telescopes, the Milky Way towers in the southern skies, flanked by the Large and Small Magellanic Clouds, at right. Although there are several thousand stars and the plane of the Milky Way all visible to human eyes, the most distant objects we can see all lie far beyond our own home galaxy. (Credit: ESO/Z. Bardon (www.bardon.cz)/ProjectSoft (www.projectsoft.cz))
This diagram shows the Earth and Moon, as well as the distance between them, to scale. Two observers located on opposite sides of the Earth at the same time, one seeing the Moon rise and one seeing the Moon set, would see the Moon’s apparent position shifted by about 1.9 degrees relative to one another. This allows us to infer the Earth-Moon distance. (Credit: NASA/JPL-Caltech)
The Moon and planets sometimes occult stars, demonstrating that stars are farther.
When one astronomical object occupies the same line-of-sight as another, an occultation will occur, as the “closer” object blocks the light that would otherwise be visible from the “farther” object. The Moon occults all of the other planet; the Moon and planets occult background stars, revealing the relative distances between them. (Credit: Bob King/Stellarium/Sky & Telescope)
First recorded in 964 CE, the Andromeda galaxy outdistances any object in our Milky Way.
This 1888 image of the Andromeda Galaxy, by Isaac Roberts, is the first astronomical photograph ever taken of another galaxy. It was taken without any photometric filters, and hence all the light of different wavelengths is summed together. Every star that’s part of the Andromeda galaxy has not moved by a perceptible amount since 1888, a remarkable demonstration of how far away other galaxies truly are. Although Andromeda is a naked-eye object under even modestly dark skies, it was not recorded until the year 964, and was not shown to be extragalactic until 1923. (Credit: Isaac Roberts)
Perhaps the most famous photographic plate in all of history, this image from October of 1923 features the great nebula (now galaxy) in Andromeda along with the three novae that Hubble observed within them. When a fourth brightening event happened in the same location as the first, Hubble recognized this was no nova, but a Cepheid variable star. The “VAR!” written in red pen was Hubble having a spectacular realization: this meant Andromeda was an extragalactic object, located far beyond the Milky Way. (Credit: Carnegie Observatories)
By that time, many more distant objects had been observed.
Spirals, initially recorded as faint, fuzzy objects with no discernible structure through more primitive telescopes, were clearly observed since the mid-1800s to be prevalent in the night sky. But their nature was a mystery, and a democratic attempt to settle the issue in 1920 only raised more unanswered questions. It wasn’t until 1923, and the identification of individual stars within one of them (Andromeda), that their extragalactic nature began to be understood. (Credit: ESO/P. Grosbøl)
The Triangulum galaxy, recorded in 1654, is our farthest naked-eye object.
The spiral galaxy Messier 33, shown as imaged by an amateur astronomer with X-ray data from NASA’s Chandra overlaid in pink, is also known as the Triangulum galaxy: a faint galaxy visible in the southern skies. First recorded in 1654, it is the faintest object visible by typical, unaided human eyes. (Credit: Optical: Warren Keller; X-ray: NASA/CXC/SAO/P. Plucinsky et al.)
In 1779, spiral galaxy Messier 58 broke that record.
