October 4, 2005: Intricate wisps of glowing gas float amid a myriad of stars in this image of the supernova remnant, N132D. The ejected material shows that roughly 3,000 years have passed since the supernova blast. As this titanic explosion took place in the Large Magellanic Cloud, a nearby neighbor galaxy some 160,000 light-years away, the light from the supernova remnant is dated as being 163,000 years old from clocks on Earth. This composite image of N132D comprises visible-light data taken in January 2004 with Hubble's Advanced Camera for Surveys, and X-ray images obtained in July 2000 by Chandra's Advanced CCD Imaging Spectrometer. The complex structure of N132D is due to the expanding supersonic shock wave from the explosion impacting the interstellar gas of the LMC. A supernova remnant like N132D provides information on stellar evolution and the creation of chemical elements such as oxygen through nuclear reactions in their cores.
When viewing objects in space, one must realize that the speed of light is a finite quantity, and that many objects that we are observing with high-powered telescopes, like Hubble, are extremely far away. If we refer to the speed of light as an unchanging value, and state that nothing can go faster than this speed, we can then use the term "light-second," "light-minute," "light-hour", and so on up to "light-year" as finite quantities of distance that are equal to the distance that light travels in that amount of time.
Based on the speed of light and the distance from Earth to the Sun, we can say that the Sun is 8 light-minutes away from the Earth and vice-versa. If the Sun showed a flare, it would be visible on Earth 8 minutes later. If an object is seen in the Large Magellanic Cloud (LMC), it takes 160,000 years for the light from the LMC to reach us. If some event occurs in the LMC, like a supernova, astronomers on Earth viewing the supernova going off today know that the supernova actually exploded 160,000 years ago. If our telescopes show that 3,000 years have passed since the time of the supernova, based on the presence of ejection material in the remnant, the actual clock-time of when that event occurred based on our Earth calendars was 3,000 + 160,000 years ago, or 163,000 years ago. Since similar objects are at various distances from Earth, astronomers usually remove the light-travel time to the object when talking about the age or when an event occurred.