Most, if not all, dedicated PDAF sensor and main imaging sensor based AF systems struggle to work when pointed at the night sky. The sole exception is the moon, which provides a large enough area of brightness and enough detail/contrast for AF systems to do their thing. Even with the Moon, it can be carefully manually focused more accurately than most AF systems will do it.
Stars and planets are neither large enough nor bright enough to give AF systems enough information to go on (unless one is using a very long focal length lens or telescope for some of the larger planets when they are closest to Earth). Even if AF systems could focus on stars, their margin of error is usually too great to give the kinds of results most people desire when doing astrophotography. The same is true of infinity focus markings on lenses that have them - they're not accurate enough for critical work with point sources of light such as stars.
For night sky shots here is what I do. Once my camera is set up on the tripod I enable live view and set my lens to manual focus. I start with the brightest star in the sky, point my camera at it and center it on the screen. I do a rough focus and then repeat at x5 and x10 magnification (I'm not sure if the 450D has this). That will get the focus near enough that some dimmer stars that may not have been visible in live view before now will be. Re-point your camera to a dimmer star and carefully refocus (x1, x5, x10). Everything in the sky except the Moon should be as sharp as your lens is capable of at this point. For shutter speeds I use a rule of thumb of 600 divided by effective focal length (include your crop factor if applicable). When using a 17mm focal length I can expose for around 30 seconds and the stars will appear motionless when viewing the entire scene. At a 100% crop the stars will appear as very short trails. By the time I'm at 640mm (200mm x 2X extender x 1.6 crop factor) I'm down to less than 1 second for shutter speed and push the ISO up to compensate. With night sky shots in relatively dim (light pollution free) skies, your exposure level choice will determine how many stars are visible. Only the brightest stars will appear at lower exposure levels and each successive exposure level will increase the number of visible stars in the shot.
Do note that the best critical focus of the Moon, which is a mere 250,000 miles from the Earth, will be slightly different than the best critical focus of even an outer planet such as Jupiter, which ranges from 365 million miles when opposite the Sun and thus high in the sky in the middle of the night to as far as 601 million miles when it is exactly opposite the Sun from Earth. Stars are obviously much further from Earth than Jupiter, but due to the logarithmic nature of change in distance as a focus ring is turned from the minimum focus distance to infinity the difference in focus movement is less between Jupiter and the stars than between the Moon and Jupiter.
In the first image below, focus was carefully set on Jupiter, which left the details of the Moon a tad soft. Compare that to the second image when focus was centered on the Moon itself. Note that both exposures were way too short to show any stars in the field of view. If we expose for the stars, the moon and Jupiter will appear grossly overexposed, as in the third photo exposed twelve stops brighter than the first two. All three were shot on the same night with a Canon EOS 7D + Kenko C-AF 2X Teleplus Pro 300 + EF 70-200mm f/2.8 L II IS. The first two were subsequently cropped further before being resized for web viewing.