Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Every observation out into deep space is also a look back in time.
Whenever you observe an object, you aren’t viewing it in its present state.
When one of Jupiter’s moons passes behind our Solar System’s largest planet, it falls into the planet’s shadow, becoming dark. When sunlight begins striking the moon again, we don’t see it instantly, but many minutes later: the time it takes for light to travel from that moon to our eyes. Here, Io re-emerges from behind Jupiter, the same phenomenon that Ole Rømer used to first measure the speed of light. (ROBERT J. MODIC)
Instead, we’re held back while light travels through space.
As shown here, the International Space Station flies over a spectacular aurora on display in Earth’s atmosphere. At its cruising altitude of around ~400 kilometers, the light we receive from the ISS here on the surface of the Earth is ~1.4 milliseconds in the past compared to events happening “now” on Earth. (NASA / INTERNATIONAL SPACE STATION)
Visible artificial satellites appear as they were ~1–2 milliseconds ago.
Whether seen with the naked eye or with modern telescopes, the light from Uranus takes approximately 159.6 minutes, or 2 hours and ~40 minutes, to reach Earth. Uranus, which has a prominent lunar system and its own rich set of rings, is the most distant naked-eye object in the Solar System. (ESO)
The two sun-like stars, Alpha Centauri A and B, are located just 4.37 light years away from us and orbit one another at between the distances of Saturn and Neptune in our own solar system. Even in this Hubble image, however, they are simply oversaturated point sources; no disk can be resolved. Proxima Centauri is approximately 0.2 light years away, only 4.24 light-years from Earth. (ESA/HUBBLE & NASA)
The closest stars, in Alpha Centauri’s system, are ~4.3 light-years away; there, it’s early 2016 on Earth.
Canopus, normally visible from the Southern Hemisphere, is seen here from the International Space Station. Bluish-white in color, the second brightest star in the night sky is much farther than either the brightest (Sirius, at 8.6 light-years) or third brightest (Alpha Centauri, at 4.3 light-years) star, located 310 light-years away. It is intrinsically much brighter than either one. (NASA / ISS EXPEDITION 6)
The three bright stars of the Summer Triangle: Vega (at top), Deneb (at left), and Altair (at right), contain seven fascinating and easily visible deep-sky objects from our perspective. The faint outline of the plane of the Milky Way can be seen passing through this collection of stars that dominates the summer skies. Vega is a mere 26 light-years away; Deneb is about 100 times as distant at 2,615 light-years. (NASA, ESA; CREDIT: A. FUJII)
The Carina Nebula, with Eta Carina, the brightest star inside it, on the left. What appears to be a single star was identified as a binary back in 2005, and it’s led some to theorize that a third companion was responsible for triggering the supernova impostor event that caused a series of brightenings in the first half of the 19th century. (ESO/IDA/DANISH 1.5 M/R.GENDLER, J-E. OVALDSEN, C. THÖNE, AND C. FERON)
The constellation of Cassiopeia is familiar to casual skywatchers as a big “W” in the sky, but in truth the constellation contains many thousands of stars that are fainter and impossible to resolve without astronomical equipment. The farthest naked-eye star of all, V762 Cassiopeiae, can be found above the first “V” in the “W” shape, as annotated. (A. FUJII; ANNOTATIONS BY E. SIEGEL)
A map of the nearest globular clusters surrounding the Milky Way’s center. A few of the globular clusters within our Milky Way, such as Omega Centauri and the great globular cluster in Hercules (Messier 13) are visible to the naked eye, but there is a visible globular cluster that’s located farther away than all of the others. (WILLIAM E. HARRIS / MCMASTER U., AND LARRY MCNISH / RASC CALGARY)
Messier 3, a globular cluster located 33,900 light-years away, as seen through a 24″ telescope. The stars within this globular cluster are approximately 11.4 billion years old, and it can be seen with the naked eye under ideal viewing conditions. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA)
The location of globular cluster Messier 3, near the Big Dipper (top) and Arcturus (bottom). If you look at the star to the right of Alkaid, Cor Caroli, Messier 3 is located along the straight line connecting Cor Caroli to Arcturus. Messier 3 is the most distant naked-eye globular at an estimated 33,900 light-years away. (E. SIEGEL / STELLARIUM)
Galaxies outdistance all other visible objects.
The largest galaxy in the Local Group, Andromeda, appears small and insignificant next to the Milky Way, but that’s because of its distance: some 2.5 million light years away. Easily visible under dark skies, it’s one of four galaxies external to our own visible with the naked eye, along with the Magellanic Clouds and Triangulum. (SCIENCETV ON YOUTUBE / SCREENSHOT)
The Triangulum galaxy might not be as massive or impressive as ourselves or Andromeda, but it’s the farthest object from Earth visible with the naked eye, and the third largest galaxy in our local group. At 2.73 million light-years away, they would not be able to find any evidence for the genus ‘Homo’ on Earth. (ROBERT GENDLER, SUBARU TELESCOPE (NAOJ))
This dual image shows the emission of GRB 080319B, imaged by Swift’s X-ray Telescope (L) and, later, followed-up by the Optical/Ultraviolet Telescope (R). This was the brightest gamma-ray burst ever recorded at the time, and was so bright that, for about 30 seconds on March 19, 2008, it was visible to the naked human eye. (NASA/SWIFT/STEFAN IMMLER, ET AL.)
Gamma-ray burst GRB 080319B was visible for ~30 seconds on March 19, 2008.
The distant galaxy EGC 1305123, seen in optical light (L) and ionized carbon monoxide (R), as imaged by Hubble. This galaxy is comparable, in many ways, to an early version of the Milky Way, and its light comes to us from about ~8 billion years ago, comparable to the most distant gamma-ray burst visible with the naked eye. (ESA/HUBBLE & NASA; TACCONI ET AL. (2010), NATURE 463, 781)
7.5 billion light-years away, its light predates Earth’s existence by ~3 billion years.
An illustration of the young solar system Beta Pictoris, somewhat analogous to our own Solar System during its formation. The Earth and Sun formed roughly 4.5–4.6 billion years ago; an object whose light must travel for 7.5 billion years before reaching our eyes would have no inkling of our planet’s existence for another ~3 billion years. (AVI M. MANDELL, NASA)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
