Image copyright NASA/ESA Image caption The birthday image features a giant cluster of about 3,000 stars called Westerlund 2

The Hubble Space Telescope has celebrated its silver anniversary with a picture featuring a spectacular vista of young stars blazing across a dense cloud of gas and dust.

The "Westerlund 2" cluster of stars is located about 20,000 light-years away in the constellation Carina.

Hubble was launched on Space Shuttle Discovery on 24 April, 1990.

Engineers expect the observatory to keep operating for at least another five years.

"Even the most optimistic person to whom you could have spoken back in 1990 couldn't have predicted the degree to which Hubble would rewrite our astrophysics and planetary science textbooks," commented Nasa Administrator Charlie Bolden.

"A quarter of a century later, Hubble has fundamentally changed our understanding of our Universe and our place in it."

'Better with time'

Famously, a flaw was found in the telescope's primary mirror soon after launch that blurred its images.

A smart fix was then installed by spacewalking astronauts in 1993 that allowed its instruments to correct for the aberration in the reflecting surface.

Four further servicing missions later, Hubble is in rude health and technically a far more capable observatory than when it was first put in orbit.

No more repair visits by astronauts are planned, but the latest assessment of its likely longevity is very encouraging.

In the past, Hubble has suffered from degradation in its six gyroscopes - the spinning devices that allow it to point very precisely at objects on the sky.

However, the current batch has experienced just the one failure - in March, 2014.

"The instrumentation on Hubble has been getting better with time, in the sense that we've been able to calibrate it better and know more about how the observatory is working," said Ken Sembach, interim deputy director at the Space Telescope Science Institute in Baltimore.

"It's also worth noting that two of the instruments repaired on the last servicing mission - the Advanced Camera for Surveys, and the Space Telescope Imaging Spectrograph - are both working five years hereafter the servicing mission, which is longer than they worked originally with their original electronics.

"And so we have great hope that these two instruments, along with the two that were installed - the Cosmic Origins Spectrograph and the Wide Field Camera 3 - will continue to be operational for several more years."

In pictures: 25 years of Hubble

Image copyright x Image caption Space Shuttle Discovery carried the Hubble Space Telescope into orbit on 24 April, 1990

See some of the best pictures produced by Hubble

It is difficult to overstate Hubble's scientific contribution.

Before the telescope launched, astronomers did not know whether the Universe was 10 billion years old or 20 billion years old.

Hubble's survey of pulsating stars narrowed the uncertainty, and we now know the age extremely well, at 13.8 billion.

The observatory played a part in revealing the accelerating expansion of the cosmos, and provided the definitive evidence for the existence of super-massive black holes at the centre of galaxies - among many, many other discoveries.

Some of the eight-metre-class ground-based telescopes can now match - and even exceed - Hubble's capabilities in certain fields of study.

These facilities have pioneered the use of so-called adaptive optics, which allow them to correct for the distortions created by looking through the turbulent atmosphere - something a space telescope like Hubble never has to contend with.

Image copyright NASA/ESA/HFF Image caption Hubble is using the massive galaxy cluster Abell 2744 as a natural zoom lens

Where Hubble remains peerless is in going deep, looking far across space - and therefore far back in time - to see the very first structures to form in the Universe.

Among the telescope's greatest contributions are undoubtedly its Deep Field observations, where it has stared at a patch of apparently blank sky for days on end to reveal the presence of thousands of very distant, extremely faint galaxies.

The latest iteration of this programme now accounts for the major fraction of telescope time.

It is called Frontier Fields, and the project requires Hubble to stare at six huge galaxy clusters.

It can use their gravity as a kind zoom lens to see what lies beyond, even further into the distance.

Jennifer Lotz, the Frontier Fields lead at STScI, explained: "They act as natural telescopes, magnifying and stretching the light of distant galaxies behind those clusters. And so by combining the power of Hubble and its very deep imaging with these natural telescopes, we really are able to see deeper into the Universe than we would without these foreground clusters."

In so doing, Hubble is now probing objects that are 10-50 times intrinsically fainter than any seen before.

The goal is to reach back as far as possible towards the time when the first stars and galaxies were forming, just a few hundred million years after the Big Bang.

And this work will neatly hand the baton to its successor - the much bigger James Webb Space Telescope.

Due for launch in 2018, JWST is being tuned specifically to investigate this early epoch. And it will do in hours the tasks that currently take Hubble many days to complete.

Image copyright NASA Image caption This full-sized model shows the scale of the James Webb Space Telescope