class MassObservations(models.Model):
    time = models.DateTimeField()
    star = models.ForeignKey('Stars')
    property = models.ForeignKey('Properties')

    observatories = (('1', 'London'),
                     ('2', 'China'),
                     ('3', 'United States'))

    station = models.CharField(max_length=2, choices=observatories)
    mass = models.FloatField()

    class Meta:
        unique_together = ('time', 'star', 'property', 'station')

import uuid
from django.db import models

class MassObservations(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    ... rest of your fields ...

OperationalError: (1054, "Unknown column 'user.id' in 'field list'")