Virginia, Galileo's oldest child, was born in Padua on 12 August 1600. Her mother, Marina Gamba, was Galileo's housekeeper. When Galileo moved to Florence, in 1610, he took Virginia and his other daughter, Livia (1601-1659), with him, leaving his son Vincenzio (only four years old) with his mother for a few years.

After he had settled in Florence, Galileo decided to put his two daughters in a convent for life. It took several years to make the arrangements. Not the least problem was that the girls were too young to make this important decision for themselves. Through the offices of Cardinal Maffeo Barberini, one of his admirers, Galileo obtained dispensation on this score, and in 1613 both girls were placed in the convent of San Matteo in nearby Arcetri, where the abbess was the sister of the secretary of the grand duke of Tuscany. Virginia took the veil in 1616, choosing the name of Sister Maria Celeste, and Livia followed the same course a year later, becoming Sister Arcangela.

Little is known about the life of Sister Maria Celeste until 1623, but about 120 letters to her father, written from 1623 to 1634 have survived. From these the picture of a loving daughter, always solicitous of her father's well being, emerges. Her letter to her father of 21 November 1623 is typical:

Most Illustrious Lord Father,

I cannot rest any longer without news, both for the infinite love I bear you, and also for fear lest the sudden cold, which in general disagrees so much with you, should have caused a return of your usual pains and other complaints. I therefore send the man who takes this letter purposely to hear how you are, and also when you expect to set out on your journey I have been extremely busy at the dinner-napkins. They are nearly finished, but now I come to putting on the fringe, I find that of the sort of which I send you a sample, a piece is wanting for two dinner-napkins: that will be four braccia. I would be glad if you could let me have it immediately, so that I may send you the napkins before you go; as it was for this that I have been making such haste to get them finished.

As I have no cell of my own to sleep in, Sister Diamanta kindly allows me to share hers, depriving herself of the company of her own sister for my sake. But the room is so bitterly cold that with my head so infected, I do not know how I shall remain well, unless you can help me by lending me a set of those white bed-hangings which you will not want now. I would be glad to know if you can do me this service. Moreover, I beg you to be so kind as to send me that book of yours which has just been published, so that I may read it, for I have a great desire to see it.

These few cakes I send are some I made a few days ago, intending to give them to you when you came to bid us adieu. As you departure is not so near as we feared, I send them lest they should get dry. Sister Arcangela is still under medical treatment, and is much tried by the remedies. I am not well myself, but being so accustomed to ill health, I do not make much of it, seeing, too, that it is the Lord's will to send me continually some such little trial as this. I thank Him for everything, and pray that He will give you the highest and best felicity. And finally, with all my heart, I greet you in the name of me and Sister Arcangela.

From San Matteo, the 21st of November 1623

Your most affectionate daughter Sister Maria Celeste Galilei

If you have collars to whiten, you can send them.

The convent of San Matteo was very poor. The nuns did not have the wherewithal to feed themselves and keep the buildings in repair. Maria Celeste wrote to her father that the bread was bad, the wine sour and that they ate ox meat. Galileo helped repair windows and personally took charge of keeping the convent clock in good repair. Maria Celeste often had to appeal to her father for help, and she was chronically ill. She bore her ill health with dignity and courage, and managed to be a great comfort to her father. She worked constantly to mitigate the difficulties between Galileo and her brother Vincenzio.

In 1631 Galileo bought the villa "Il Goiello" in Arcetri, near the convent. From this house he could see San Matteo and hear its bells. It was here that he spent his final years under house arrest. Part of the sentence that Galileo received in 1633 read as follows: "As a salutary penance we impose on you to recite the seven penitential Psalms once a week for the next three years." [cite] Sister Maria Celeste took it upon herself to perform this penance for him. She died, however, on 2 April 1634, less than four months after Galileo's return to Arcetri.