Three short lines ask a question that sounds scientific but quickly becomes something more tender. How can we tell when a star has died? The answer, tucked into parentheses like a whispered aside, is that stars "leave their lights on." The light we see may have travelled so far that the star itself is already gone, and nothing in the sky tells us.
The speaker seems quietly wistful rather than despairing. The image of leaving the lights on is homely, almost domestic, like someone stepping out of a house and keeping a lamp burning. That ordinary phrasing makes a vast idea feel intimate. The poem suggests that loss can be invisible: something can look present long after it has ended. Because the poem is so short, it also leaves room for readers to think of people or things they have lost, while the light of them still seems to shine. The mood is hushed, reflective, and gently sad, with a small spark of comfort in that lingering glow.