WASHINGTON — Ground-based radio telescopes last month detected a supernova explosion in the nearby M82 galaxy that scientists say is the closest supernova discovered in the past five years.

The object, dubbed SN 2008iz, would have been visible even to amateur telescopes were it not for the dense gas and dust surrounding the exploding star, which left it invisible in every part of the spectrum except the radio wavelengths.

M82 is an irregular galaxy in a nearby galaxy group located 12 million light-years from Earth.

Despite being smaller than the Milky Way, it harbors a vigorous central starburst in the inner few hundred light-years. In this stellar factory more stars are currently born than in the entire Milky Way.

M82 is often called an exploding galaxy because it looks as if it is being torn apart in optical and infrared images as the result of numerous supernova explosions from massive stars. Many remnants from previous supernovas are seen in radio images of M82, and a new supernova explosion was long overdue.

Astronomers have been waiting to catch the next big blast for more than 25 years and had started to wonder why the galaxy has been so silent in recent years. In the end, it took a little digging and looking in the right wavelengths.

The discovery was made when Andreas Brunthaler of the Max-Planck-Institut fur Radioastronomie in Bonn, Germany, examined data from April 8 with the Very Large Array (VLA) of the National Radio Astronomy Observatory, an interferometer of 27 identical 25-meter telescopes in New Mexico.

“I then looked back into older data we had from March and May last year, and there it was as well, outshining the entire galaxy!” Brunthaler said.

Radio emissions can be detected only from core collapse supernovas, where the core of a massive star collapses and produces a black hole or a neutron star. It is produced when the shock wave of the explosion propagates into dense material surrounding the star, usually material that was shed from the massive progenitor star before it exploded.

But observations of M82 taken last year with optical telescopes to search for new supernovas showed no signs of this explosion.

The supernova is also hidden on ultraviolet and X-ray images.

The supernova exploded close to the center of the galaxy in a very dense interstellar environment, which could explain why M82 has been silent for so long: many of these events may actually be something like underground explosions, where the bright flash of light is covered under huge clouds of gas and dust and only radio waves can get through.

“This cosmic catastrophe shows that using our radio telescopes we have a front-row seat to observe the otherwise hidden universe,” said HeinoFalcke of the University of Nijmegen and an affiliate with the Netherlands Foundation for Research in Astronomy, or ASTRON.

By combining data from the 10 telescopes of the Very Long Baseline Array, the VLA, and the Green Bank Telescope in the United States, and the Effelsberg 100-meter telescope in Germany, using a technique known as Very Long Baseline Interferometry, scientists were able to produce images that show a ring-like structure expanding at more than 40 million kilometers per hour, or 4 percent of the speed of light, typical for supernovae.

The team estimates that the supernova exploded in late January or early February 2008. Only three months after the explosion, the ring was already 650 times larger than Earth’s orbit around the sun.

The discovery will be detailed an upcoming issue of the journal Astronomy & Astrophysics Letters.