The firmament is not as we see it, but

[Tachyon son]

Regarding the stars in the sky at night, I have read a lot of times about the "false" image we get from it. Mainly due to gravitational lensing and stars that no longer exists altough their light is still there.

But, how is it possible then that, as I far as I know, the constellations are shown in their actual shape from the earth? Are they free from the effects formerly mentioned??

Probably the question sounds naive, but I was not able to sort it out through Google.

 

[mgb_phys]

Constellations don't really have a shape - they are just the 2d view from Earth of chance alignments of stars. The stars aren't connected in any way and can be at very different distances. Seen from any other point they wouldn't look like that.

Other than that, yes we see stars in the past. Light travels at 300,000 km/s which is fast but if a star is billions and billions of km away then it will have taken the light a long time to get here.
In practice the stars you can see with your eye are quite close ( less than a few 100 light years) , so you are only seeing them as they were a few decades to a few centuries ago. They haven't changed much in that time.

But with a large telescope we can see objects as they were billions of years ago, these objects are probably long gone now - we are just seeing the light that left them when they were young and has been traveling across space to us.
It's like being a long way from lightning and hearing the thunder long after the lightning strike has finished

 

[Nabeshin]

Gravitational lensing really doesn't play a part in the stars we see in the night sky (be they with a telescope or with the naked eye). The only time gravitational lensing becomes noticable is when we're talking about clusters of galaxies lensing the image of a quasar or some other such scenario. Certainly not commonplace, though.

 

[Chronos]

We see the light from stars as they appeared hundreds, or thousands, or millions, or billions of years ago. The more distant ones have greatly changed by now. Lensing is irrelevant, as Nabeshin noted.