Mjolnir, as displayed in the Avengers, would probably weigh 50-60 pounds. We are, for the moment, precluding any magical enchantments that make it unliftable, immovable or able to alter its gravitational constant via the power of the Odin-given worthiness enchantments.
Its head is twice the size of an ordinary sledgehammer (whose head is slightly smaller than the average brick) and weighs about 18-20 pounds. Since it is supposedly comprised of a metal more durable and at least as heavy as iron or steel, we can approximate its unaltered, non-magical specifications.
Thor's hammer is supposed to be a long handled warhammer. As legends have it, during its manufacturer, its handle was somehow reduced in length (likely some trick or machination by Loki) so its head is disproportionate to its handle length. Given Thor's incredible strength that didn't seem to matter much.
Sledgehammers for human use rarely weight more than 10-20 pounds, since generally they are used as tools. To scale one up their normal size to the size of the head of Mjolnir would make its physical weight approximately 50-60 pounds, and make it impossible for an unaltered human being to wield no matter how strong they were!
Even if you were to remove the enchantments for worthiness, its physical weight and very short handle would preclude normal humans from doing much more than picking it up.
With the new request of the hammer being made of collapsed star matter...
Blah, blah, blah; Asgardian hyperbole at best. If it were, just supposing it was, there is no place it could sit on Earth that would be able to support it, effectively. I am assuming it was created as a magical artifact in Dwarven forges of enchanted metal that resembles steel or iron.
If it IS actually made of collapsed star matter, then there is no where he could set it down that it would not collapse that object (like the helicarrier, for instance) like a tin can. A teaspoon of neutron star can weigh 900 times the mass of the Great Pyramid of Giza! For the record, the Great Pyramid of Giza weighs 5.9 million tons.
Since we have seen Mjolnir sitting on tables, lying on city streets, Thor standing in office buildings, the hammer sitting someplace in the SHIELD helicarrier without THOR holding it up, we can be safe assuming it is not weighing more than the Great Pyramid of Giza. We can assume it may be more massive than its apparent weight but being truly made of ONLY collapsed stellar matter, even the lightest of such neutronium-like material would cause the hammer to be untenable in any human environment.