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For some reasons I do not understand the stars look very blurry in this photo:

enter image description here

Here is the photo details

enter image description here

I have tried different exposure times such as 10" or 8", but the outcome is still the same.

I have set the autofocus to manual. The photo looks like I did not focus properly. However how can I focus if the stars are infinitely far away from us?

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    \$\begingroup\$ Stars are not stationary. They are moving, so you can't use just any shutter speed. 15", 10" and 8" are all a little too slow for a 34mm lens on a crop camera. Look up how to use the "500 Rule". photo.stackexchange.com/questions/30263/… \$\endgroup\$
    – Mike Sowsun
    Commented Apr 27, 2020 at 1:22
  • \$\begingroup\$ @MikeSowsun More accurately said; the planet that we are on when photographing the stars is rotating ( and orbiting around its host star ) relatively quick compared to movement of our galaxy and the expansion of the universe. This is giving the appearance that the stars are moving faster then they do as a result of the universe expanding. If the earth was not rotating at all then you could have a 2minute exposure and stars would not appear to have moved because they are millions/billions of miles away. Focusing is another matter entirely. \$\endgroup\$
    – Alaska Man
    Commented Apr 27, 2020 at 18:19

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The photo looks like I did not focus properly. However how can I focus if the stars are indefinitely far away from us?

Indeed.

For stars, you need to manually focus.

How to do it with a modern DSLR / MILC:

  • First, find some very bright star
  • Select the live view mode
  • Aim the camera at the very bright star
  • Zoom in the live view (not the lens!) as much as you can so that you can accurately manually focus. By this I mean use the camera's digital magnification button, not turning the zoom ring of the lens.
  • Select the manual focus mode on the lens
  • Turn the focusing ring of the lens very carefully to make the bright start as point-like as you can
  • Recompose the shot as you wish without touching the focusing of the lens anymore
  • Take the picture with proper exposure

Note that some focus-by-wire lenses may respond differently to slow and rapid movements of the focusing ring, i.e. turning it very little abruptly may be different from turning it the same amount extremely slowly. At least for canon RF lenses, you can change the behaviour of the focusing ring from the camera menu. Not all lenses support this, though.

Edit: some additional notes:

  • Lenses are usually not parfocal. So set the zoom ring position first and only then focus, not vice versa.
  • If the camera has exposure simulation, you need to have correct exposure set before you see the very bright star in the live view mode.
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    \$\begingroup\$ it is also possible to focus on some distant light (e.g. a city or street light) if no star is bright enough to show up on the LCD screen \$\endgroup\$
    – jng224
    Commented Apr 26, 2020 at 18:02

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